When Do I Change The Filter?

[dids100]

I have started a new 2' tank, did a fishless cycle for a couple of weeks, had 5 neon tetra's in it for about 3 weeks now and 4 plants. I've tested the water weekly and the nitrate/nitrite levels have remained very low, so when do I need to change the filter?

One of the neons died last week and I dont know why. Another doesnt look well (lost its colour/looks very weak).
I had a snail problem which has been treated and is why I have delayed putting in anymore fish. Could the tetra have been affected by the treatment or should i automatically change the filter even if the water tests ok?

 

[andywg]

you only change the filter media when it is literally crumbling apart. Don't worry about that. Neons can be fragile and are prone to Neon Tetra Disease.

As an interesting aside, most chemical snail treatments rely on heavy metals, which a number of water dechlorinaters neutralise, rendering the treamtent useless.