I dont have any discus yet but I had a similar experience when I switched from a sub-par reactor to a duffuser. I lost 6 cardinals, 4 sae, 5 bloodfin tetras over 2 days. I dialed back the co2 and added 2 powerheads. 1 is just above the diffuser and flowing down the back wall and the other is at the top pointed at the surface. When using a glass diffuser, water cicrulation is important. You wanna make sure that you have enough current to get the bubbles all over the tank and enough surface agitation for gas exchange. Then I slowly turned up the co2 while watching the fish for any strange behaviors. So far everything is doing good and the plants are pearling like crazy. I have very soft water out of the tap so I add a gh booster to help against ph swings. With the addition of co2, you may find that your limiting factor with plant growth will now be nutrients. I would also suggest you look into a c02 drop checker from ebay if you dont already have one. With all the variables in planted tanks, its nice to know exactly how much co2 is actually in your water.