Backyard Aquaponics
A place to discuss aquaponics

I've had an idea for a heat-sinking pond for growing trout over the summer.

The pond would be 1m diameter and 2m deep, giving a total volume of around 1500 litres. I'm thinking that in the heat of summer, the ground at 1-2m depth would still be less than 20 degrees, and the surface area of the water (exposed to the 45-degree heat) would be much less than the area exposed to the cool ground.

Our local hardware store is selling 1m offcuts of industrial plastic pipe in 900, 1000 and 1200 mm diameters for $50, $60 and $70. I'd be thinking of buying 2x 1m lengths, and then either attaching a circle of pond liner to the bottom using silicon, and sealing the join between the two pieces, or else filling the bottom 10cm with waterproof concrete and sealing the joins with bitumen.

The digging of the hole would be a killer - rather like digging a well!

It'd be a weird geometry for a fish pond, but I don't suppose it would be a huge problem?

I've got a spare cool room and a 20ft freezer container that I was going to put a ft in. I haven't eaten any of my fish so I'm thinking trout would have to be over summered or given away. My pool is 2mtrs deep but the larger area allows it to heat up.

If you can find cement pipes I've seen pictures of them making wells by getting inside and removing dirt until it starts to sink down and another ring is placed on top.

I'm not sure how well sealing pond liner to pipe with silicone would work though the idea of geo-coupling with the ground does have a moderating effect on system temperatures. Biggest challenge is that the grow beds have an even greater effect on system temperature. Also, surface area affects aeration and trout like lots of aeration.

Maybe to keep them over summer it might mean using another form of bio filter and growing duckweed to keep temps down.

Yeah, They're not going to be eating much anyhow, stressed as they are with the higher temps.
Perhaps if well aerated and not fed much they would make it.
Duckweed will convert some ammonia directly too.

My attempt to do something similar was to insulate the system and have a buried tank, the main tank is about 1200L and the buried tank is 1000L. I use the underground tank to pump into the main system during the hottest part of the day. I only had it running for the end of the summer last year so I am not sure if it will be enough but I survived last year by rotating 3L ice blocks from the freezer every 3-4 hrs on the hottest days. Also my trout reached 26c with no obvious signs of stress, but I had plenty of air, water circulation and they had time to acclimatise.

Thats the key

Well, we eventually built a trial system at a mate's place using two of these massive (1m diameter, 1m long) fibreglass pipe off-cuts, sunk into a 2-metre deep hole in the ground and glued together in the middle. Digging the hole was a BIG job. (You know you've done well when you need a ladder to get in and out of the hole!)

Grow-beds consisted of 8x half drums (44 gallon) plus a bath-tub, all with auto-syphons. Uniform distribution to the growbeds was tricky but was roughly achieved via a short horizontal piece of stormwater pipe with holes drilled along at uniform height, and poly pipe connectors screwed into the holes. In hindsight, a single growbed is much simpler.

Sadly, the unforeseen problems of (a) young kids dislodging growbed hoses, and/or (b) an intermittent leaking seal at the bottom (where we had glued a big plastic base on the pipe), meant that the water levels dropped lower than the pump's ability to lift, and all 50 rainbow trout died well before the system's summertime performance could be seen. The system is now sitting idle as the owners have moved interstate (and are renting out their house).

I maintain that the basic idea of a very deep, buried tank for thermal buffering is a good one, and the reasons for this system's failure were quite separate. However, had we made it to summer, the above-ground drums we used as growbeds (which were black) would have raised the water temperature significantly.

This is an interesting idea, where did you get the pipe Jimmy?

The bottom of my tank is 6 foot underground and even with temps over 100 F I have a hard time keeping the water temp above 85F.

Burying things a few meters in he ground is not going to provide thermal stability.

http://www.backyardaquaponics.com/forum/viewtopic.php?p=342557#p342557

That is not true. We can argue how much effect it has, but to say it does not provide any thermal stablity is wrong. Ever seen a water pipe buried underground burst? I never have, but ever pipe above ground on my place busted last year. So to say it has no effect is BS. Also I have 4 fish tanks, the one buried in the ground stays about 85F the ones above ground go into the mid to high 90's. So at least 10 F higher than the in ground tank. I am willing to bet anything the results are the same this winter, except the above ground tanks getting colder than the inground tank.

In fact one of my above ground tanks got over 100, so I had to do a water change, meanwhile the inground tank was still at 85.

I didn't say it has no effect. Read what I said, burying something a few feet underground does not provide thermal STABILITY. In order to have an unchanging temperature i.e. completely stable, the depths required are in the order of many tens of meters. You may achieve some insulation by burying components shallow, which means temperature variations are less than the surrounding environment, but any temperature variation is not stable. Did you bother to read the information in the links? The models in those papers allow you to compute the temperature variations against different depths given an environmental temperature gradient. They also show the depths required to achieve thermal stability.

In order to provide a suitable climate for fish which can be sustained year round, accounting for any extreme in temperature, exceptionally hot and exceptionally cold in the same year, requires greater depths than can safely be achieved in the backyard setting.

Please check your facts against well documented research and read what others have actually written before you accuse people of telling untruths; you could upset some people otherwise.

You said it will not provide thermal stabiltiy and that statement is FALSE. Buring a tank even at ground level with help it resist temperature changes, and that is fact.

Definition of stabiltity: resistance to change, especially sudden change or deterioration

The OP did not ask about complete thermal stability. The title is heat sink and it will help.

I know my facts, you where responding with something the op did not ask. And yes going a few feet underground will help to sustain fish that would die othewise. I will prove this over this winter. The bottom of my fish tank is actually about 10 feet below natural ground level, since my green house is going to be partially underground.

Again no one here asked about complete thermal stabilty.

Interesting stuff, thanks Bunson. However, the Nofziger paper was dealing with seasonal temperature variations, in which case yes, you have to go a lot deeper to find inter-seasonal "stability" (i.e. finding somewhere that stays the same temperature all year round). Shallower ground (on the order of 1 metre deep) reaches what we might call a "quasi-stable" temperature in that it doesn't reach the daily highs and lows experienced above the surface, but instead sits more around the daily average. Then it slowly changes as the daily average changes with the seasons.

In our trial system, we were not interested in maintaining the same temperature all year round, just avoiding water temperature getting too high for trout during summer heatwaves. To that end, a shallow system was satisfactory. However as I said above, the system failed for other reasons and in fact we might have found that the above-ground growbeds absorbed too much heat anyway.

(If you live somewhere that the average summertime or wintertime temperature is likely to stay outside the happy range for the fish for long periods, then this system wouldn't be likely to work in any case without supplementary heating/cooling.)