Backyard Aquaponics
A place to discuss aquaponics

Looking at a 2nd hand poly rainwater tank as an FT.

It's more expensive than an IBC but it is cylindrical- which I understand is better for water flow...

Any reason why this wouldn't work? Tank is 1.4m diameter and 2m high - so about 1,800 liter capacity depending on how much I end up cutting it down or whether it's possible to cut it in half to make 2 X 1,000 lures tanks...

Thanks in advance!

The circular shape should help you get good water circulation and hopefully "sweep" solids toward your SLO (if you use one). At least that's how it works in my two fish tanks - though mine are re-purposed corrugated aquaplated steel, raised garden beds so a bit more like an oval with flattened sides.

Cutting it in half would work well because 2 m deep might be a bit tricky if you have to do any maintenance on things in the bottom... or netting your fish. But not sure about what impact cutting it would have on its structural integrity.

Is it corrugated or smooth walled? Corrugated can be a bit trickier when it comes to plumbing... if you have to do anything beyond what's already there. I had to order suitable tank fittings online but I imagine you'd find them easily in Sydney.

Both my FTs are poly water tanks, as is the ST on my large system. Corrugated is easy, just buy the washer to fit corrugations if you need to take a pipe through a corrugated section.
2m is too deep for a tank that small, almost impossible to clean, or see the bottom unless your water is crystal clear. Also very difficult to catch the fish!

Low profile tanks are better- my small system 4500l tank is only about a metre deep and the 9500l one is about 1.5m deep.

I'm planning a CHIFT PIST system with SLO.

The tank I'm looking at is smooth-walled. I'm hoping to cut in half as I really don't fancy using a step ladder to look after the fish.

Does anyone have experience with cutting a poly tank down?

Would it really affect the structural integrity?

only real issue will be structural / strength - that will depend on the build.
The main issue will be lack of rigidity when cut in half so the top can flex or deform.

Everything else is a plus - if it was me I would go ahead and if needed consider a frame to ensure it keeps its shape.
Frame does not have to be complicated.

I think 2x 1m deep FT is a good outcome if the top is suitable to become a base.

[edit] angle grinder will do it. But be careful as the tanks have a bit of weight.

It will be quite wobbly, but that's no problem if you have it in the ground, and the ground is stable enough not to collapse the sides in. I left 3/4 of my big tank top in, to maintain rigidity, and the small one has a ~20cm ring of the top still in place with the middle cut out, plus a stainless steel cross-brace.

However, it isn't really ideal, corrugated sided tanks are much more rigid with an end removed.

I cut a 5,000 litre tank in half, upended the top, blocked all the holes and made two tanks out of it. Worked well so far, see my 'regretfully not updated in ages system thread' below. All I would do different would be to wrap a lot more insulation around them before I put the mini orb steel around. By the way that steel wasn't really needed to strengthen the walls it was an attempt to make the system look better, something you wouldn't need to worry about if your still planning to keep the tanks in the shed. I think they make a good fishtank, plenty of people use them.

I forgot to mention earlier, a jigsaw does a great job of cutting poly tanks- then use a sharp knife to clean up any burrs etc, for a nice neat finish. An angle grinder tends to melt and burn the plastic, leaving sharp edges.

True, I started out with an angle grinder but the fine dust just stuck to everything and was very hard to clean up, no such problems with the jigsaw.

cut a small piece (3 inch or so) from the tanks, a ring if you will, can then heat that with a heat gun and stretch it over the top of the tank, then secure.
Wont be perfect, but adding a second layer to the top will help stop the warping, a metal ring would be better if you can be bothered.

All excellent suggestions thank you!

Especially handy to hear from those who have experience cutting the tanks as I totally would have just attacked it with the angle grinder.

Gunagulla - I'm having trouble picturing how you braced your small tank... Is it possible for you to take a photo?

Plenty of pics in my system thread, the first one, on the first page.

Ahhh ok. Makes sense now! Is that the top of the tank? So it's one low profile water tank and you just cut a hole in the top slightly smaller than the actual circumference of the tank?

Would it be possible for me to do the same pole bracing even though I won't have this same "ring"??

Yes, I just cut a hole in the top of it. Yes, pole bracing would be a big help, even without any reinforcements. Untreated wood or stainless steel would be best, even a couple of sawn off broom handles would probably do.

Check out my AP thread. The tank is 2.5m diameter and about 1.6m high. It was a factory seconds (warped lid) and the manufacturer offered to plastic weld a 50mm irrigation pipe around the top to stop it wobbling around. That was an additional $120 and well worth it in my opinion. I'm going to install some LEDs to make it easier to see the bottom and I have a large pool rake which should make harvesting a cinch (in theory). I really don't see any major downsides to the poly tank - provided you find a cheap one.