Help brown type sludge on filter

[Sleepwhatsthazzz]

I'm new to owning goldfish as an adult. They are honestly causing me so much heartache. We had to watch one die during the week and it was awful.

Now I've noticed a brown type sludge on filter of the tank. It is a new filter, about a month old, replaced after another fish died and I noticed the old filter was also brown. Is this what is killing my fish ? How do I solve this, what is it? I don't want my other fish to die. I will try to upload some photos. It also seems to be gathering on ornaments on one side of the tank which is a new thing. Help! I don't want to be killing more fish

[Sleepwhatsthazzz]

Bump