If you do not want fish,
Remove carbon and use the bacteria we talked about once a month.
Its bit our of my experince I have never had a system without a fish.
Bacteria need source of food which usually come from fish food and waste. I do not want to recommend put food just to creat access nutrients because you will add all sort of other unwanted nutrients.
My understanding is the biological filtration adjust to your system and eventually reach a balance of sustaining enough bacteria that the nutrients in the system can sustain, ither bacteria die and what left is usually what nutrients can sustain.
Bacteria needs carbon, phosphorus and other nutrient sources.
So, I can only imagin that you need to add these food sources one way or another...
For now pets remove the carbon and reduce water changes to twice a week or even more and see if you can creat a balance. If this is diatom, once balance is achieved it should turn in to green algae.
Couple of question:
- When you first started your system did you go through a brown algae phase followed by green algae phase?
- what's your equipment exactly? Do you run skimmer?
- can you take coule of pictures of the system and post it?
- what else do you dose?
Kudos to the exchange of information I am reading in this thread.
I run many systems fishless. Get some interesting shrimps and/orCrabs.