I have a 55g aquarium cycled for about two years. I was using 2 Fluval C4 hob filters, and I had frequent issues with one of them, so decided to get a canister filter. I got the Penn Plax 1200. I installed the Penn Plax last Friday. I decided to keep using one of the C4 for the time being, and I took the biomedia (Matrix) I had in the other C4 and in a box filter that I kept in the aquarium if I needed to start a quarantine, that I placed in the Penn Plax with a lot more of new Matrix for the top tray (I also have Purigen in there). I also have 2 trays full of new biohome (another biomedia that I'm using for the first time). The bottom tray is sponge and floss. I probably have close to 10x more biomedia than before, albeit most of it is not cycled. But I didn't take out any cycled biomedia out of my previous setup. I also added Sachem Stability to help colonizing the new media. So everything was fine until yesterday (Tuesday). In the morning, I noticed that 3 ember tetras and 1 lemon tetras were dead. I had to go to work, but measured ammonia which was at 0. When I came back from work, 5 more embers were dead. I tested: ammonia 0, nitrite 0, nitrate 20, ph 7.2. Not knowing what to do, I did a 50% water change and added more Seachem Stability. I also decided to run carbon (which I've never done) in case there was some toxicity in my water that I could not detect. I also turned off the air stone I had and lowered the flow on the Penn Plax (and moved the direction of the diffuser), since the only other change I could see is that I have more water circulation than before, and I'm wondering if they died of exhaustion. This morning: my remaining 3 embers and 4 lemons dead. Coming from work: 1 lemon and 2 glowlight tetras dead. So I decided to further slow the water flow. My aquarium was well stocked: about 105% according to aqadvisor. I have corys, platys, neon tetras, gold tetras, glowlights, endlers, danios, and now 1 remaining lemon tetra. I never had any issue in the last 2 years. I tested: ammonia 0, nitrite 0, nitrate 10. All my fish appear fine. That said, I'm now worrying about oxygen, since I don't know if I have enough water circulation. Any idea what's the problem and what I should do? Sorry for the long post, thanks for reading.