1 Canister Filter for 2 4 Gallon Cubes

Never tried it, but you would need a perfect balance to avoid the problem you talked about. It's not about how much "power" you have. It's path of least resistance. Say something were to obstruct the intake of one tank. The pump will still suck up the same amount of water overall, only now it will be getting it all from one tank. Now the tank with the obstructed inlet will still be getting its normal outlet volume and overflow. The other tank will be removing twice as much water as it was before, and still only getting it's original inlet volume, and will eventually run all but dry. I'm sure there is a way to use float valves of some sort to create a safeguard, but at that point it becomes overcomplicated and cost prohibitive. Further, in the chance one tank has an issue such as an illness, both will be affected.

I'm a fan of building new things and innovating but this seems like it could be very dangerous to try. The problem could be solved but it depends on how much time money and complications. Your willing to deal with to get one filter on two tanks that won't spill out in your house and kill one or both tanks. If your going to do it I would do very thorough testing on whatever setup you come up with for several weeks, blocking intakes putting a lot of solids thought the pipes anything g and everything that could go wrong before you ever put fish in it. This is one of those things where there is a much simpler solution..... Two filters

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Small submersible powerhead filters? Basically a powerhead with a media section attached at the bottom. I use one in my 10 gallon for secondary, but would probably do fine by itself. I don't know what brand mine is. Its a cheapo with no markings. About 2"x2"x4", adjustable flow rate, also came with another section that housed a carbon filter that I don't use, would have added another 2 inches to the length. I put it horizontally in the back with some decor in front, and other than the cord which is ran right along the corner, you never see it. I have seen other ones simmilar to it at my lfs.

I'd get a small canister for each tank...a small Eheim Ecco or its analog would work. The tubes can easily be adapted to lilly pipes.

Mark

Yeah the only way I know of to run more than one tank on one filter is to have the tanks in series with the intake in one and the return in the other with exchange between the tanks completing the flow cycle. There are two ways to pass the water between the tanks, one is with a siphon between the two tanks at the same level and the other is to have an overflow like you would use with a sump setup.