Yes indeed! There's something going on with your filtration and it's bugging me. All filter bacteria need are:
- A substrate to settle on eg. foam, ceramics or gravel.
- A constant supply of oxygen, normally provided by water movement.
- A food source ie. ammonia or nitrite.
- A hospitable environment ie. moderate pH and hardness. They need some minerals to function.
Now we know you have the first three covered. I wonder if your water is hard enough but in most of the country that's not a problem. Do you have a domestic water softener? It's a longshot but worth ruling out. These things can be a problem as they remove calcium ions fom the water and replace them with sodium ions - a disaster for aquatic life. If you have the kind that needs salt to recharge this may be the problem.
I would try to clean your filter as little as possible. If it will run for a couple of months between cleanings then let it get nice and grubby, as long as you're keeping the bottom of the tank clean and for this you're best using sand, a piece of slate or the bare glass and siphoning off any droppings and uneaten food as often as you can. Basically, keep the tank clean and the filter dirty.
