Hi Gavin. I read through all the responses. Like some have said, excess nutrients are a main culprit but there are others such as leaching from substrate and/or rock. You have two distinct problems, algae and bacteria. Chemi-Clean or Red Slime Remover are safe when directions are followed and absolutely work. A phosphate reactor is, in my opinion, essential. Make sure to use GFO (Granulated Ferric Oxide) as the medium, not Aluminum. Stop all the water changes or you will disrupt the whole system. If you don't mind a little Green plant in the tank, I would recommend some Halameda to suck up the unwanted nutrients. I would also recommend a small bottle of Prime and use as directed.