Are you saying that the reactor is connected before the filter, before any pre-filtering? The Reactor 1000 is very good and I would recommend that you place it after the filter. I know some people think that you get better solubility of CO2 by placing it before a canister filter but I would avoid that if you haven’t any way of pre-filtering it to remove any particulates that could clog your reactor… CO2’s saturation point in water is so high that before you ever got CO2 bubbles in your reactor your fish would likely be died or showing sights of great distress. The bubbles are likely O2 if your plants are periling. Again, you have a great reactor that will do just fine after the canister not before. Just my opinion... I agree that the 2215 should be fine.I have an Aqua Medic Reactor 1000 connected to the intake of an Eheim 2215. I noticed that sometimes the reactor fills with what I assume is CO2. Is this okay? Or is this a sign that the pump is not strong enough?