I just finished reading nolapete's 4300 Gallon Plywood Build and after thinking about all the problems people are having with sealing the tanks with various coatings I wondered why no one uses PVC sheets to build a tank inside the tank.
Everyone seems to trust the material to be used in the plumbing of the filtration systems, so my guess is that it's fish safe. It wouldn't have to be structural just hold water. Plus you could solvent weld the seams (I'm thinking lap joints) which almost everyone who's done PVC plumbing is familiar with. Silicon sticks to it well, so that would be another option for joints. It's also paintable in contrast to many other plastics, so that you could easily paint it to whatever color you like after installation. My only real concern would be it's rigidness. It's not nearly as flexible or elastic as other plastics. I think if you filled a tank with cold water (the colder the plastic the more brittle it becomes) the PVC might crack instead of stretch under the tension produced by the water pressing it up against the frame.
So here's how I envision how you could have a fool-proof waterproofing of a wood tank.
Build a stacked dimensional lumber frame similar to nolapete's. Yes, I think it's overkill. But it's about as simple to do as wood working gets and there's something to be said for the peace of mind that comes with knowing your frame is solid. Build a "flimsy" PVC container around a disposable frame that has your desired inside tank dimensions and spot holders for where you want to put your viewing windows. Drop this into the wooden frame an suspend it from the top edge it leaving a gap of 1-2" all the way around and on the bottom. Run your plumbing through the walls of the PVC and the wood. Finally fill the cavity with a spray foam product (for big tanks I was thinking something like this).
After curing I think the foam would be rigid enough to withstand the water pressure of a full tank (in fact the disposable frame inside would have to be strong enough to hold up against the pressure of the expanding foam). And It would add the fool-proofing to this design. Because even if you had a leak in the PVC it would never make it past the polyurethane foam. It would also help with keeping the water temperatures constant and reducing heating/cooling costs that can really mount up on large aquaria.
Finally when the foam has cured, silicone you viewing windows to the PVC.
The issues that I can see right off the bat:
PVC rigidity/brittleness-> PVC sheets seem to contain plasticizers as opposed to plumbing components. These make them more forgiving. Plus the foam would be more forgiving than lumber I imagine.
Plasticizers leaching into the water-> I asked an organic chemistry major what he thought. After he looked into it, he said that the plasticizers are chemically bound to the PVC, and that after filling and draining a couple times over the course of week almost all the chemicals on the surface of the PVC that would go into solution in the water would have. After that no worries.
Polyurethane foam not holding up to the pressure-> I think that the right type of foam will need to be found that is hard enough to withstand the pressure yet flexible enough that it maintains a waterproof seal to all surfaces.
Polyurethane foam might be toxic to fish-> My initial thought is that I doubt that it's highly toxic. And even if there are concerns, it never touches the water except on the possibility of the leaks that it's sealed. And those surfaces should be negligibly small.
I'm sure I've overlooked something in this, that's why I'm posting it. I'm hoping there are people out there with experience with some of these materials. Let me know what issues could be expected in this sort of build. With any luck they won't be anything that can't be handled. I'm a long ways away from being ready to start a build of my own but I figure it can't hurt to get as much of this figured out before hand.
Everyone seems to trust the material to be used in the plumbing of the filtration systems, so my guess is that it's fish safe. It wouldn't have to be structural just hold water. Plus you could solvent weld the seams (I'm thinking lap joints) which almost everyone who's done PVC plumbing is familiar with. Silicon sticks to it well, so that would be another option for joints. It's also paintable in contrast to many other plastics, so that you could easily paint it to whatever color you like after installation. My only real concern would be it's rigidness. It's not nearly as flexible or elastic as other plastics. I think if you filled a tank with cold water (the colder the plastic the more brittle it becomes) the PVC might crack instead of stretch under the tension produced by the water pressing it up against the frame.
So here's how I envision how you could have a fool-proof waterproofing of a wood tank.
Build a stacked dimensional lumber frame similar to nolapete's. Yes, I think it's overkill. But it's about as simple to do as wood working gets and there's something to be said for the peace of mind that comes with knowing your frame is solid. Build a "flimsy" PVC container around a disposable frame that has your desired inside tank dimensions and spot holders for where you want to put your viewing windows. Drop this into the wooden frame an suspend it from the top edge it leaving a gap of 1-2" all the way around and on the bottom. Run your plumbing through the walls of the PVC and the wood. Finally fill the cavity with a spray foam product (for big tanks I was thinking something like this).
After curing I think the foam would be rigid enough to withstand the water pressure of a full tank (in fact the disposable frame inside would have to be strong enough to hold up against the pressure of the expanding foam). And It would add the fool-proofing to this design. Because even if you had a leak in the PVC it would never make it past the polyurethane foam. It would also help with keeping the water temperatures constant and reducing heating/cooling costs that can really mount up on large aquaria.
Finally when the foam has cured, silicone you viewing windows to the PVC.
The issues that I can see right off the bat:
PVC rigidity/brittleness-> PVC sheets seem to contain plasticizers as opposed to plumbing components. These make them more forgiving. Plus the foam would be more forgiving than lumber I imagine.
Plasticizers leaching into the water-> I asked an organic chemistry major what he thought. After he looked into it, he said that the plasticizers are chemically bound to the PVC, and that after filling and draining a couple times over the course of week almost all the chemicals on the surface of the PVC that would go into solution in the water would have. After that no worries.
Polyurethane foam not holding up to the pressure-> I think that the right type of foam will need to be found that is hard enough to withstand the pressure yet flexible enough that it maintains a waterproof seal to all surfaces.
Polyurethane foam might be toxic to fish-> My initial thought is that I doubt that it's highly toxic. And even if there are concerns, it never touches the water except on the possibility of the leaks that it's sealed. And those surfaces should be negligibly small.
I'm sure I've overlooked something in this, that's why I'm posting it. I'm hoping there are people out there with experience with some of these materials. Let me know what issues could be expected in this sort of build. With any luck they won't be anything that can't be handled. I'm a long ways away from being ready to start a build of my own but I figure it can't hurt to get as much of this figured out before hand.
