Why does my internal filter keep clogging?

I have a Fluval 2+ in my 90l tank.

It keeps clogging up, the yellow indicator goes down and I have to keep washing the media to get it going again...this is happening every 2-3 days just now.

Heres a short history of whats been happening lately in the tank

• outbreak of Ich currently, with heat + salt treatment
• two shoals of guppy fry
• possibly a dead fish in there somewhere, cant find it but no ammonia spike
• very heavily planted tank
• first time it happened replaced the impellar
• UGF in tank as well

I really cant work out why its clogging. When I wash the sponges theres a little (couple of bits) of plant matter and they are a bit 'thick' with matter, but IMO might not be enough to clog, so I'm not sure if its that or not?

Tonight when I washed it I put pantyhose round the outside so only finer matter will get through - is that the best solution?

Opinions?:help:

will my pantyhose idea help / work?

Thanks bghill, will do that if it clogs again, so far the pantyhose going well but only day 2, so next step clearly the impellar housing - how did you clean it?