I've had my 20 gallon tank set up for about 4 months and it was cycled and has had a male betta and five snails living in it for about three months (since the tank was fully cycled). I've been having a problem with green water, and I was doing frequent large water changes (about 50%- 75% once a week) trying to get rid of it. Well, I tested my tank just now for the first time in a few weeks (irresponsible on my part I know) and now my tank is reading at 0 ppm nitrates, 0 ppm nitrites, and 0.25 ppm ammonia. I have a prefilter sponge and biomax in my filter that havent been changed or washed which should have BB so I don't know why my cycle was broken. Does anyone know what happened and what I should do to fix it?