"I was actually gonna ask if I could somehow run the co2 into the outflow of my filter? "
That's ironic, I was just reading some planted tank articles where the writers were saying that CO2 injection ahead of the cannister impeller has become their newest preferred choice. They were using Inline Reactors.
I don't know exactly what makes a good -inline- reactor (ie. one that won't slow the pump speed too much.) (I assume this is all way too much for a 5 gallon but its interesting to know about none-the-less.) There are people that make non-inline reactors by pumping tank water into a tube with a power-head, forcing it into a cylinder they've made. The cylinder is open at the other end but has a grid or something so that a sponge can be trapped at that far end. An airline injects the CO2 at the beginning of the cylinder and there are 2 or more Bioballs (those plastic thingies used in some filters for biomedia). The strong water-current spins the bioballs and breaks up the co2. The sponge keeps the co2 trapped to be further broken up but lets the water go on through.
Supposedly these reactors are often more efficient (give higher ppm of co2 per amt of co2 gas used) than diffusers(but it depends on the diffuser and there is controversy about this). I read about these in context with gas cylinder systems.
Sorry if I'm way off base, just thought you'd find this interesting given what you were saying up there... (true planted tank people could no doubt put this in better perspective for you!)
~~waterdrop~~