I have a central sump system that holds approx. 1200 gallons. It is a holding system for our fish and inverts, consisting of many glass aquariums, acrylic cubicles and fiberglass tanks. We are always adding on, upgrading and improving, but for the last two months we have been fighting a .25 - .3 ppm Nitrite level, verified by several different tests. Thats not much but isn't acceptable either. I have tried over and over to clean, fix, aireate, bio-boosters you-name-it. Nothing has worked. We have emptied the system of all inhabitants several times in hopes the biological bacteria would catch up but no. And the strangest part is this sytem is about 9 months old. After an intial cycling, very normal ammonia spike followed by nitrite then measurable nitrate, we stocked it with fish and they were healthy, and happy. They stayed in the system for a month or two before being sold. But now, we just can't get rid of this pesky nitrite after having zero readings for months.

Was my nitrogen cycle insufficiant?

Does the water temp going from 68 to 86 over the last 9 months (system is outdoors) have anything to do with it?

Should I bleach it and start over?

Lack of substrate affect it?

Should I build a refugium?

The Gritty:

Filters: Home Made Bio-tower with three sprayers 5' x 14" round filled with spriflo and bio balls initially seeded with some bio balls from my 75g at home, 55 watt UV, zeolite activated charcoal, filter sock, homemade protien skimmer 4' x 6"

Pumps: Two 3500 gph submersable water pumps, one 1/8 hp regenerative blower (I think it could be bigger but dissolved oxygen levels are very good)

Treatments: Have used three brands of bio-bacteria additives, occasional half dose of pima fix to prevent disease, am-quel plus if we must stock the system with fish, and to prepare new water.

Feeding: little as possible (cannot ship fish with food in their bellies)

Just my $.02:
you're under the .5ppm max...
Frequent 25-50% water changes

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55gal ~ Freshwater
90gal ~ Saltwater
100gal ~ ?