Frehswater refugium?

I am pretty sure java moss needs light, which it won't get in a canister filter. You can buy a breeders box and put it in there. Maybe even put some in a hang on back filter. Just put a screen preventing it from falling over.

Good idea if someone has puffers and can get a breeding population of snails going in the refugium.

I am not sure but it is worth a try. You can also try feeding less or changing your feeding routine. For example if you use floating you could switch to sinking. I am not sure what you have in your tank or the size but if I had to guess it is overstocked. This will be your biggest problem.

You probably could still do water changes as often as liked, but plants need nutrients.

Just FYI many plants don't incorporate as much nitrate into their mass as you may think (so choose wisely). I recently attended a presentation for an unpublished study in marine aquaculture that found (through mass balance) that the plants they were using only removed a fraction of a percent of the nitrates from the system. However, they got a lot of nitrate reduction from the denitrifying bacteria that had taken up residence in the root structure (rhizosphere) of the plants. These bacteria colonize surface area in environments with an absence of oxygen to eliminate nitrate. The roots were in submerged planters with some sort of substrate. Keep in mind you will also need a bioavailable carbon source to feed the bacteria, but supplementation is not always necessary.

Pothos are the way to go, check out the thread in the DIY Filters forum.

Pothos will help. They will need a little light (ambient or a 60 watt bulb) and they can't be 'smothered' under water as they are terrestrial or buried in a chamber.