I usually put a layer of plastic bag or use a bowl to break the flow of water when topping up a tank with new soil, it prevents the dust from stirring up in the tank for new setup.
Most aquasoil like HELP aquael, MR aqua, ADA, tropica have these problems if you are not careful.
My first tank with ADA 12 years back had this problem, because I didn't know to do it properly. Took several weeks with large water change to clear up the water. I didn't have live stock at that time, so 50% WC per week and with canister cleaning. I put 3 stacks of fine filter floss (eheim) but it still took longer than expected.
Best advice is still to start right.
Bookmarks