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In a 5 gal
tank you would be able to get away with one small growing fish or smaller species of clown. A pair would be best suited in a 10 or 20 gal
tank depending on species. The puffer would be out for a 5 gal
The fluval 1 could definately be used for water flow but I would remove the media. In smaller tanks, any amount of trapped detritus will affect water quality quickly. I would instead suggest about 10 lbs of live rock and some quality oolitic aragonite sand (1-1Â½") to function as your primary filtration source.
The need for a heater will depend on the ambient room temp where the tank is set up. There are many small titanium submersibles that could be tucked behind the LR
out of view.