Do you have a HOB filter? If so check the filter media. I had some small flies (a little bigger than fruit flies) that were always around and in my 55g driving me crazy. After about a month of them being around I was doing some routine maint. and when I went to rinse the filters they were full of the tiny maggots. I didn't want to replace the filters but it was the only way to get rid of the flies. They haven't reappeared and it's been a year now.