I have a betta in a 1.5 gallon tank. Gravel dirty so bought a gravel cleaner. Before cleaning the nitrite level was at 1 which made me nervous. So I cleaned the gravel and changed about 1/2 the water and retested the water when all done, right after. The nitrite level is now 2!!! Why would it be higher? I just cleaned the gravel and changed 1/2 the water. What do I do? Thanks!