I have been busy setting up new equipment on my 250l discus tank and today I got as far as putting the external CO2 reactor in. Well the existing flow order was: inlet into filter (straight through bottom of tank,it's got drilled pipework.), then output through UV filter before returning to tank. So, on autopilot I plumbed the co2 reactor after the uv filter before the water returns to the tank. Well that didn't work, as the flow rate was right down to no use at all! So my thinking was this: I've replumbed the UV and the CO2 reactor on the inlet from the tank. Now gravity and water pressure are driving the water through the UV and the CO2 reactor. The filter pump has simply to return the water straight to the tank. Bingo, healthy flow rate restored! What I wanted to check was that there is no dangers with doing things in this order? I know putting the UV before the filter has no issue, as the UV only kills pathogens in the water as it goes through the steriliser, and will not affect the bacteria in the filter itself. (might slow down the cycling of a new filter, but this is not relevant as I have an established filter.) My concern was whether there was any issue with putting the co2 reactor before the filter, as now the co2 injected water is going through the filter before returning to the tank. Could this affect the biological workings of the filter at all? Any thoughts appreciated.