Hi again,
What a wonderful gesture Goldie, you have a heart of gold and you picked the right name.
Hi Fishnut,
This is the thing with fish keeping. Things can happen so quickly that if you blink you miss them, and only when your tank is truly established can you relax a little.
It's like a scale on a fine balance, and the slightest thing can change that balance, sometimes to the good, and sometimes to the bad.
When something of this nature happens you have to sit back and go over EVERYTHING that you did over the previous week or sometimes month, and usually the thing that caused the prob will come to light.
Even the water from your tap can change from day to day, as many here will testify, so find out the situation of your tap water and monitor it occasionally to make sure you are not adding more probs to your tank.
It's highly unlikely that it 'is' the filter, but more as above... a sudden climb in toxicity, either ammonia or nitrites, or both, and you mention cleaning the gravel. Now if you disturbed this unduly (especially if you have overfed a few times) then you could have created a bacteria explosion that might be part of your prob.
You'll have the answers tomorrow with Goldie's kits.