still cycling?

[tufa44]

Hi everyone. I'm a newbie saltwater aquarist. Just bought a second-hand setup in January. It's a 54 gallon corner tank with overflow but no sump. I have a Rena XP3 canister running from the overflow and an in-tank Tunze skimmer. There is about 40 lb of live rock. The tank came with and still has a maroon clown, a cleaner wrasse, a red flame hawkfish and a blueface angelfish. I sold one queen angelfish because I didn't think it was appropriate for my setup. I also added a purple anemone for my clown. There are assorted inverts and corals also.

My problem is that I still have nitrate problems and diatoms. All the fish, inverts and corals are healthy. Ammonia and nitrites are 0 but I'm still measuring at or just under 10 ppm nitrate and <=0.3 ppm phosphate, pH consistently at 8.5, calcium about 500. Do you think my tank is still cycling or is this a phosphate problem? I am using tapwater but hope to invest in an RO/DI system soon. Thanks!