Will it be difficult to measure nitrates (ammonia and nitrites also) with one small fish (4 inch lab) in a large tank (75-90 gallons)? In other words would the volume of water dilute the readings to a "non measurable" level?
My daughter has 4 small peacocks in a 75 gallon tank which was seeded with a mature sponge. No ammonia-nitrites-nitrates after a few weeks. Since the fish are feeding and producing waste, is the volume of water enough to dissipate any toxins, sort of like a large body of water? How long will it take for the level of ammonia to rise to a level that BB start to feed and convert? Can nitrates be there but not measurable with the kits available? Fish are fine.
If the filter was properly seeded you will not see an increase in ammonia or nitrites. You should however see an eventual increase in nitrates. If you are using the API test kit make sure you shake up bottle #2 very well. I like to turn it upside down and bang it on the counter.