It is protein and normal water changes will not get rid of it because it
floats on top. There is a special filter called a skilter that may take
care of it for you. I keep salt water tanks and the skilter works good
for them but having never kept a brackish water tank I am not sure how
well they will work for that. Check eBay for one so if it don't work
that well, you have not spent too much money.

Vicki