What fish or shrimp would be safe with a dwarf puffer in a medium planted tank?
Can you buy Refugio's for freshwater?
Also I have three danios and two live shrimp. Should thy produce enough ammonia to start my cycle
With a refugim you dont need a filter. It is a filter just make sure there is filter media in it. If you wnated you could use regular biomedia but use java moss as your mechanical media. Then the shrimp and snails would eat off it.
Puffera are pretty messy but a refugium will add water volume and reduce nitrates because of the live plants.
Ottos are usually compatable with dps. Rcs its shaky but sometimes works. I think cories would be fine to. The reason they leave ottos alone I think is because they are not open water swimmers. I also wonder sometimes if they realize the ottos are cleaning the tank up. Idk about that.
I think you're thinking of a sump filter rather than a normal refugium Refugium are supposed to be places of refuge for critters or plants that wouldn't survive in the main tank. Combining the sump and refugium into one place is doable, but you have to be careful that the snails have some way to get back into the display tank without going through a pump. Most sumps are done below the tank due to the space required, so they're not suitable as true refugiums. Usually it's recommended to have the sump hold 20-50% of the water volume of the main tank for proper filtration if it's being used as the sole method of filtration.