You never want to have less than 0.03ppm phosphates, or photosynthetic life will suffer.
It would probably be a good idea to get some more rock for your tank, both so your fish have places to hide, and so there's more space on which beneficial bacteria can grow.
It would probably be a good idea to get some more rock for your tank, both so your fish have places to hide, and so there's more space on which beneficial bacteria can grow.