The UFO Concrete Pool

By C. Moormann

Quality Fish Pictures Made Easy

Of all of my fish tanks, the UFO Concrete pool is my favorite diversion. Some days I will spend enough time staring into it that my legs have gone to sleep when I try to get up. It is a round design with the outer rim 19 feet in diameter with an inner circle/rim of about 11 feet. It has the shape of an upside-down wedding cake with the small circle/cylinder 11 feet diameter by 2 feet in depth with the top level being the bottom of top circle/cylinder being 19 feet in diameter by 4 feet deep. I have calculated the volume to 10,000 gallons if filled to the top but we usually just fill it to about 1 foot deep in the upper section so just 3 foot deep rather than the full 6 feet capable (from the bottom of the center section.

1. If you look at the detail sketch below, some of the important details are evident. The upper part of the drawing shows the concrete pieces and the way they went together. The sketch is not to scale so I will describe the dimensions -- the wall thickness, bottom thickness, and footer thickness is approximately 4 inches. The two outer vertical walls (looking like an upside-down 'T') are 19 feet apart (inner diameter) and the inner walls are 11 feet apart. The outer vertical walls are 4 feet tall and the inner walls are 2 feet tall.

2. Steps in building the hole. The first thing I did was rent a back hoe and dig out the hole to about 22 feet in diameter with the outer edges about 4 feet deep with the center about 2 feet deeper. The upper edge of the outer ring is about 1 foot above ground and one important thing is to backfill a little under the footers -- do not dig into the impacted ground and pour the footers. The entire pool needs to be able to become one piece with equally soft dirt under both rings/footers and under to bottom pours so that if there is any settling, the entire pool can sink into the ground equally.

3. Steps in building the forms. The form is similar to something you would make for a skateboard ramp. The lower part in the sketch kind of looked like a speed bump while the other side resembled a lower version of the half-tube the skateboarders use. The inner layers (that touches the concrete) was made of 1/2 inch waferboard and was painted with motor oil to help keep the concrete from sticking. The rest of the framework was 2x4 and 2x2 pieces to give the forms strength. The inside arc is shorter than the outer form arc so 4 or 5 inches had to be cut off the inner waferboard (I started with 4x8 foot sheets). The base needs to be big enough to cover the width of the base footer as it is important to be able to pound some type of stakes into the ground to prevent the base from expanding when filled with the very heavy concrete. You can intentionally make the base thicker but the modifications would make it more complicated. The top can have 1x2 or other smaller boards across the top to keep the top from changing shape. The more you have, the more difficult it is to get the concrete in and tamp it down to eliminate bubbles and make a smooth top. Wood clamps work really well also and remember that after the first form is made all further pours have one end of the form overlapping the previous form and you can stake at the bottom and clamp at the top but the vertical struts must be strong enough to resist bowing in the middle (where you now can not put a clamp). I tried to show the detail for the ends of the form with two vertical 2x2s each screwed to the wafer board and then held together with other longer screws holding the two sides of the form together. For strength and to prevent cracking I used chicken wire down the middle of the form structure with some overlapping through the 2x2 end pieces. To connect the vertical concrete to the footer/bases, I carefully threaded the chicken-wire down the center line of the footer ring and added more strength by pounding re-bar stakes into the ground on the center-line. Long rebar was carefully bent into an arc and positioned about 2 inches below the top and threaded into the chicken wire and out holes in the 2x2 sandwich ends to continue into the next form. This rebar gave strength to the top of the concrete and helped to position the chicken wire in the center of the forms. In practice I staked rebar down the middle line before mixing any concrete and the first pour was a 10 foot length of the outer footer followed the next day with the first pour into the forms -- with the chicken wire sticking out of the footer. After the form was filled, I poured enough to have enough footer for the next pour. I think it took eleven uses of the forms to complete the outer ring (and several weeks). The use of a 4 foot level is recommended to set the forms properly vertical and to make sure the bottom was completely level I ran enough water into the hole to find the low/high spots and adjusted accordingly. The footer rings were about 14 inches wide and with the dirt removed for the ring -- the ring can be filled with water to check on the flatness. With the footer rings properly made, the height of the forms set the top of the vertical forms properly. At this point (before pouring any concrete) some thought must be put into how the drain/filter plumbing is made. For my pond, I had a filter sump about a foot outside of the outer ring (on the northwest of the pond center -- for 'feng shui' purposes -- more details later). The sump was made from two water softener tubs with one having the bottom cut off and carefully pressed together to form a 6 foot deep cylinder. A bulkhead connector for 2 inch pvc pipe was put into the side of the sump at the bottom with the 2 inch pipe extending to the exact center of the pond with an elbow and a vertical element coming up out above the concrete and capped during the pour. The hole for the sump was made slightly larger than the cylinder and filled with concrete around the outside -- filled with water to keep from floating. The concrete sealed the joint between the two sections of the cylinder and some old holes in them. A 2 inch 'T' was inserted with another vertical pipe and cap that came up just inside the inner ring wall. My original idea was to have an undergravel frame using eggcrate and fiberglass screen on a wood frame over most of the bottom with the gravel sloping down to the inner ring wall pipe. I planned to get in the pool and stir up the gravel and let it settle - then open the cap and let the debris flow to the sump to be pumped out on the lawn for cleaning. The center cap would be removed when covered by the undergravel frame and with the other vertical pipe capped, water would be pumped from the sump to a waterfall and would return to the sump through the undergravel filter. The pipes go through the footer rings just below the usual thickness so are held in place by the concrete of the rings until the last pour establishes the bottom. The excess vertical height is then cut off leaving just enough to put on the two caps. I had recently read an article on Oriental design and wanted to position things accordingly -- in the case that Asians take over the world, my pool would have more value if designed properly. See and others on the design of Kyoto and the positioning of gates. The city is laid out with mountains to the northwest with apparently a stream out of the mountain with all of the temples on the mountain. The gate leading in this direction was called the 'Demon Gate' and the temples had some preventative value. My plan was to have the waterfall made with ledges for ceramic temples and planters. There was going to be another outer ring about 6 feet outside the pool with 'J' bolts going up to connect to some kind of a dome over the entire pool with the six foot flat area concrete for pool furniture. The original plan ran out of funds and free labor and I have made a pvc frame covered by a transparent pool cover (making it look like a UFO). There is a hatch with a ladder leading down into the pool in the proper position of the 'human gate'.

4. Inner ring issues. The inner ring is smaller (more of an arc) but I just used my circular saw to cut the forms in half and adjusted the top and bottom for the changed arc. As seen in the drawings, the top of the inner form needs to be 4 inches above the top of the outer ring footer. Most koi ponds are made using rubber liners because most concrete pools crack. My opinion is that the upside down 'T' design of the footer/vertical joints makes a perfect seal when the floor is poured and with some soft dirt under the entire assembly, the entire mass expands/contracts/settles together to prevent cracking. The only issue we had was that without proper hammering of the forms to remove bubbles, there was some leakage through the horizontal outer ring. Painting the inside of the ring with white concrete paint/sealer took care of that problem.

5. Other issues. The sump design required a lot of bending over and if someone fell into it head first, there may be a lawsuit. I dug out the dirt in that area and made a ramp down to it so I could stand next to the sump to work on it. Later I covered up the sump and made a couple of rubbermaid tubs into undergravel filters and with these above ground, the water is pumped from the pool into the tubs and it goes through the gravel back to the pond. In practice I have had 3 or 4 three-hundred gallon water troughs just outside the pond that the water is pumped into and they are connected with screened siphons and return to the pool after going through each tub. The way to look at it is that the main pool and the cover produce a large body of warm, filtered water -- think of the pool as a locomotive to which you can attach passenger cars. The pools are covered with a wood/frame with shelf-glass tops arrangement. I put a variety of livebearers in the pools in the summer as a kind of summer crop with other fish and fry in the pools. The pools above ground are easy to work with but stay a lot warmer when connected to the pool. The pool is hard to catch fish out of -- we use a livebearer trap from Cabella's to get some out but usually have to pump out the water to an inch or two depth to really catch the fish. The cover of the main pool provides heat and keeps out the leaves and tumbleweeds that used to fill the pool before I covered it.

The UFO 10,000 gallon pool
Details on the construction of the UFO pool. Photo, C. Moormann, 2010 Click to Enlarge

The UFO 10,000 gallon pool
Details on the construction of the UFO pool. Photo, C. Moormann, 2010 Click to Enlarge

The plants in the picture is my own 'Sargasso Sea' The Sargasso Sea The plants are a large floating block of bacopa about 8 x 6 feet. The purple/pink flowers are almost always out. The plants do not touch the bottom but have a large mass of roots. Fry hide in the plants and hopefully egglayers use the roots. I presently have a dozen or so emperor tetras that I never see that may be reproducing in the plants. The pink masses on the walls and pvc are apple snail eggs -- the gray eggs are from mystery snails. Without the apple snails the entire upper floor and sides becomes covered with a layer of algae and masses of hair algae are everywhere -- there is a lot of light. The pool never freezes but unless I heat it, it falls below the 56 degrees F minimum temperature for most tropical fish. I have tropicals in it for about 6 months in the summer and move them out and put in koi and goldfish for the winter. The snails can stay in year around as well as 4 blue lobsters I have in there. The pvc supports are for the cover. The cover goes over the frame and is secured to the pool with truck tie-down strapping cinched down over the cover to the outside of the outer ring. Wind has affected the cover and another couple of rings on the top of the cover hold it down for the high winds we have here regularly. You can barely see the arm of a plastic chair in the upper left corner of the pic for when I want to sit in this unique environment. BTW the circular design was so I could set a pump going to circle the water and float in an inner tube endlessly -- but we have never found the time. I have videos of my grandkids and a couple of great danes in the pool when we originally set it up -- I have irrigation water in a 1 inch hose that will fill the pool overnight in the summer.

Here are movies of some of the fish in the UFO pool on You-tube on the internet (the frame reflected on the water makes you think you are in a large airplane cockpit) :


The pics above were taken about 1 month after we put tropicals in this year and the medium platinum mollies are only about 3 weeks old and almost 1 inch. Since (2 weeks later) then they are now 1.5 inches and there are about 500 fry per week showing up and growing fast. The water is now green.