Hi, I have had my 100L tropical tank for about 17 months now. I have 21 fish , the largest being 4 small barbs. tempreture is between 24-25 c. My local aquarium centre checked my water tests & everything was ok, apart from very slight ammonia level. The tank has run very smoothly , with no problems until now ! when i clean the filter out ( in tank water } there is hardly anything in it. The ammonia levels keep going up, i am doing a quarter water change every week now. I have real plants in the tank. theres hardly any residue in the substrate & i am very careful not to over feed the fish. So my problems are.... why is there no residue in the filter & why does the ammonia level keep going up ?
It sounds like you somehow crashed the bacteria colony. Best bet is to find some established filter media to kick start again. Or i have found thr probidio product (biodigest + biotrace) are very useful for jump start.