Why and what is this brown fluff?

I think it must be a type of mold do u do weekly water changes ? And how much do u feed your fishes ?

 

With a 600 litre tank it is totally impractical to do weekly water changes. If anything, I under feed the fish.

 

Lol you will be surprised people do although I agree its hard work my biggest tank is only 300 but I have a few tanks and do once a week water changes on all of them it gets easier once you get into a routine with it

 

I think he's talking about your normal 25% water changes, not a complete water change.

 

When last did you clean the filter? Could that not also be part of the problem?

 

I agree with @Jwh, looks like cr@p ,scientific name detritus ;-)
You do not supply much info on your tank setup and filtration, but I would say that your trickle filter isn't doing a great job, whether it is down to cleaning or else I can't say.
It might be good at biological filtration but it seems you have some spot/spots where the flow isn't adequate and that is where you will get this build up.
Clean the filter and add a powerhead or wave maker to improve flow. You may also like to add some fine sponge/floss to trap this stuff better.

 

Yes I mean a 25 percent. Water change

 

I agree with your suggestion. I will remove the pump, unfortunately the piping to the pump is fixed to the tank so it will not be a simple task. My filter comprises four individual layers of sponge but only the top two ever require cleaning.

 

I believe the problem is with the food. All my fish are mid water feeders. I buy mid water pellets and most still float on the water surface. My fish do not feed on the surface so this food ends up going over the weir. I use flakes for my small fish and they also tend to float.