I'm a beginner so I don't have much advice, but I can sympathize with your problem. I have a 10 gallon with the exact same filter. I’ve been experiencing difficulty with an ammonia spike, too. Unfortunately, mine really is ammonia and not ammonium at my ph levels. I’ve been doing 50% water changes every 2-3 days and treating new water with Prime for about a week now. I also added a dose of Tetra SafeStart when first cycled and have been adding weekly doses of “Cycle” since then. As of today my readings were:
Ammonia 1-2
Nitrites .25
Nitrates 40
Yikes! So I did another water change with Prime. Two hours later the numbers were back down to ~ .25. Even though I checked the Ammonia on my tap water 3 months ago, I went ahead and followed Lucy’s suggest and tested it again. I couldn’t believe when it turned green! I tested it three times from two different faucets, all registering around 1. Luckily, my 75 gallon is still registering 0, so I have at least 1 working bio-filter. That also tells me it is not a problem with the test kit.
Not a whole lot I can do about the water supply other than dilute with distilled. I’m also thinking this type of filter with the biogrid just can’t support a large enough colony of bacteria. I’m going to replace it with a Rena SmartFilter if I can find one. I can use the established Bio-Chem stars from my
canister filter to instantly start the biofilter on the 10g (I hope!). I’ll let you know if I see improvement with the new filter. Please let me know if you discover a trick for combating this problem.