Rainman, I do have an IBC tote that is my bio filter with bio balls and other plastic matter. This tank or "pond" what have you, is indoors. Stays right around 75-85 degrees inside all year.. I have radiant heat in the concrete and that alone heats the tanks in the winter, so that's nice. What kind if sand filter do you run? I also do about 1/4 water change on a daily basis which I believe has been keeping the ammonia and nitrite at bay. It's apparent to me that I need more oxygen to both my bio filter, along with the fish. Hetero and aerobic bacteria need oxygen just as much as my tilapi do.
My system was not designed for growth. I actually needed to inhibit growth as much as possible with minimal feedings, minimal "safe" water temperatures also.
I used several Magnum Pro non-bypassing power filters, a 40 watt UV chamber and a Pentair 9FB Fluidized Bed sand filter. Filtration was handling 150 gallons, but designed for 900+ gallons of "normal" use.
I was never a fan of bio-balls as they clog badly and the dead bacteria film inhibits new bacterial growth by covering needed substrate. the same goes for bio-wheels or any variation of them. They just need too much cleaning for me. The fluidized sand filters are self cleaning and the substrate surface area to footprint is unmatched.
I would NOT consider the solids filtration I used as a good way for solids removal, but my fish were in the basement next to the wife's office. She HATED the hatchery smell at first, but once she began seeing black ink rather than red while doing the company books, the stench in her office became MUCH more tolerable! lol
25% daily water changes are okay. I never bothered changing water daily. I Changed it entirely weekly, or if the fish showed signs of stress, then I changed it all out, twice. Partial changes never eliminate water quality issues, they only delay and briefly reduce them.
