Backyard Aquaponics
A place to discuss aquaponics

To plumb though pond liner using uniseals, all one needs is a backing of the appropriate thickness. If the tank or bed is less than 1/2" think wood, then just drill the appropriate size hole for the uniseal. I have found that the larger uniseals are harder to put the pipe through.
You can use rubbing alcohol to lubricate the pipe and fitting to make it easier to push through. Safer than windex or soap.

And a few pictures of the larger size.

Whilst I agree this is possible, could you state your reasons for using a uniseal over a nice big tough tank outlet?


the over-engineer in me wants to go tough on this one...

The important thing is to make sure the liner and backing for the uniseal are supported since if the liner or backing are being pulled, then leaks can happen between the liner and the uniseal.

In each of my plywood grow beds I have installed uniseals for the overflows and have not had a leak in any of them.

Before I had used uniseals, I would have agreed. But now that I have used the uniseals, I find they work well (they are not designed for high pressure but This really isn't a pressure situation.)

Main reason to use a uniseal instead of a bulkhead fitting is cost and ease. Price difference between a 3" uniseal and a 3" bulkhead fitting is about $15 for the cheapest bulkhead and then I would need threaded fittings on both sides of the bulkhead (which would be quite difficult to manage under the liner by the way.)

I agree that in the bottom of a large lined fish tank, I would probably not want to use a uniseal, but for this duckweed pond where the sides are only 16" high, I'm not too worried about the pressure and if it does spring a leak, I'm not killing my fish, only wasting duckweed nutrients and water.

cool, sorry to interrupt your description

It does not need to be a very accurate or circular hole? this was going to be my next negative, yet your hole seems to be a bit rough, and still works. Perhaps they could be useful for more things than I had imagined. If I ever receive delivery of them that is

You do want a nice round hole through the backing or hard tank but it doesn't need to be surgically perfect or anything. I just use a regular hole saw of the appropriate size to cut the backing hole or the whole in whatever else I might put a uniseal through.

Then I just used a pocket knife to cut the hole in the pond liner, that is a rough cut but so long as it is slightly smaller than the hole in the backing, it seems to work ok.

Is that jagged piece of liner the cut out from the hole ?

I would have thought that you would need a neatly (read exactly) cut hole so that it wouldn't leak.

I believe the seal of the liner is made by the uniseal pinching the liner between the uniseal and the backing piece. I was skeptical at first but under minimal pressure situations, (like grow beds) it seems to work just fine. I can't say that the holes in the liner for the other instances I've used them have been any less jagged. I don't know of an easy way to cut a perfect hole in rubber liner. I have installed 5 uniseals in plywood and liner grow beds, they all seem fine so far. I have 3 uniseals installed in my large lined fish tank, two above the normal high water line and one below the normal low water line for the top up valve. One uniseal for the overflow from the monster grow bed and one uniseal in the lined space under the walkway that one grow bed drains through. So far so good with them all.

Only uniseal I've ever had leak was not installed through liner but hooking a 1/2" pipe into a 1 1/2" pipe.

I'll keep you guys posted about how these two uniseals in the duckweed pond hold up as I am leak testing tonight. If the do find under 6 inches of water overnight tonight, I'll try filling it up tomorrow or at least filling as deep as I'm planning to use it.

I'll admit I'm a little worried about these two seals since I didn't use very big backing pieces and they are not supported all that well. If I were to dig out from under them and pull, the liner would stretch and probably leak.

So, What I have done there is just about worst case (well I suppose the sand I've stuffed up under it is supporting it some so letting the backing and all just hang off the liner would the the worst and would fail I'm certain) so I'll report back as to how it goes.

Not leaking as of this morning.

Well, I promised to report on progress.
The 1" uniseal through the liner is fine.

The 3" uniseal through the liner is seeping darn it. Well that is what I get for being stingy. The backing plastic that I showed in the picture actually broke while we were trying to push the pipe through the fitting. I resorted to a bucket bottom that happened to have the right size 4" hole in it from an old project. Unfortunately that bucket bottom is pretty thin and apparently way too flexible for this purpose.

So warning all, If ya gonna plumb though liner, make sure your backing for the fitting is well supported because when things flex, the liner do stretch and then can leak!!!!!

When I manage to sort out a fix for this hole I'll let ya know. It will probably come down to draining the pond and then see if I can put the uniseal with backing onto the pipe end backwards with a backing inside the pond. Otherwise I'll probably have to pull that bit of pipe out all together and go get a bulkhead fitting and secure a section of sturdier backing to the pond frame. I just hate trying to screw fittings in down under things in the dirt...........

'tis one of my pet hates too... the dirt gets in the thread, you can't see what you're doing, crap gets in the pipe. It's just misery all around, but once in forever in

I've had bulkhead fittings leak too. If there is any off pressure on them, it can cause the same problems.
Either one through liner needs a good backing and support to keep it from leaking.

interesting... I'll keep that in mind, but I've never had any probs with my bulkhead through rough cut pond liner

Just my 2 cents from my experience.

TC, I think you already know that the leak is not the uniseal, but the liner. Not so much the lack of support for the fitting etc.. The support is mostly necessary to have something to push against when installing the uniseal, almost for certain you would have the same situation with the bulkhead. I have found, as you mention, that a good support is necessary, but not for the fitting so much as to give the uniseal something to grab (the backing for the liner where the uniseal will go), the leak is not based on the uniseal, but the strength of the liner, or on the amount of liner the uniseal has to grab ahold of. I have used the cheap 6 mil black plastic as a liner, and it will hold no problem over a large unsupported area, the plastic would have to pull loose or tear. It is very unlikey to tear in any thing we are doing, and if the uniseal goes in, correctly, it will not pull loose either. Let me try to explain.

I have actually put in the uniseals in standing water, with very little loss of water (top of pipe was above water line). You must have enough support on the backing to be able to really push the pipe through the uniseal. I managed to get my hand underneath the liner and when the uniseal and pipe were lined up ready to push, then I punched a hole into the liner from the bottom and pushed the uniseal and then the pipe in. The liner will stretch as you push, and the whole will open to the "necessary" size for all to work. This ensures that the liner is past the bottom of the uniseal. When all is in place, if the under side is going to be visable, then I may trim off any excess liner that pushed through. If the backing (in your case it was plywood) has the correct whole size, then I find that the liner cannot pull lose. If the presures are beyond the capacity of the liner that is another issue. By pushing some of the liner through the whole with the uniseal, it does wrinkle a little, however, this excess actually serves to better seal it up.

To summerize, when putting a uniseal through a liner, everything you did looks great.... except, don't bother to cut the whole in the liner until you have the uniseal pushed at least part way through the whole. Then, only put a small cross shaped cut in the liner to allow it to tear just enough to allow the seal to push through. This will ensure that you get "plenty" of liner through the whole and will NOT pull loose.

Oh, one other thing.. make sure the backing, what ever it is, is the correct thickness. If it is too thin, then the uniseal does not have enought to grip. If it is to thick you will not have a problem..........unless there will be reverse (pushing up or back) pressure on the seal (that would not be likely in our applications). If your whole is cut right on the backing, then you will need soap or alcohol to get the pipe through, if the liner passes all the way through with the seal, it won't slip out.

Sorry if this sounds like I am going over it to much, just trying to be clear.

I hope this helps.

Mathew