Quick filter cleaning question - am I doing it right?

[treehouselover]

I have a Fluval filter. Can't remember the exact model, but it was from Pets at Home and I've had it about 6 weeks. It flips forward and two panels containing sponges pull out. There's also a bit in the middle full of white balls.

Whenever I do a water change, I wash the two sponges that pull out, but they're never dirty. I never get even a slight bit of plant or anything off them. I would have thought that they'd be full of crud and when I rinse them they'd come cleaner.

I'm worried now that all the crud is still inside somewhere and I'm not cleaning properly. The opening where the water comes out is a bit green and slimy and I occasionally wipe it with kitchen roll when the water is low during a change, but other than that i don't remove any bits at all.

The tank looks clear, but I have just lost a fish, and I lost another one about 3 weeks ago.

Are the white balls smooth or rough?

How big is your tank and what fish do you have in there?

When you say you rinse the sponges, what do you rinse them with?

Have you got recent water test results?

Hi eau

Tank is 70l. I have 5 neons, one guppy, one plec and one adolescent platy (sole survivor from babies that were born about 2 months ago). I have lost 2 platys in the last 3 weeks.

I'm reluctant to get any new fish now because I want to make sure the tank is right first.

The white balls looked rough. I've been googling and I'm pretty sure it's a Fluval U2. I've also found the instructions although they are for U2/3/4.

I wash them in the old water that I've removed.

I tested ammonia tonight and that was fine. I will do all the others tomorrow. I did a water change tonight so wasn't sure there was much point doing it straight after?

When you changed the old filter for the new one, how exactly did you do it? Did you keep track of water quality after you put the new filter in?