Grover;4760528; said:I used GE Silicone II. 100 % silicone & no antibacterial ingredients that I could see. I was pretty sure on other threads this had been used for aquariums by other people with no problems. I did use the black but again others have used this with no problem. I agree the sump is the only change I've made so something in the new sump is causing it. I am currently doing a 60% water change to see if this will help. Thanks for your ideas.
If you used Bathroom and Kitchen Silicone it has antifungal additives in it. This may be the problem. If you used Window and Door then the Silicone is not the problem.
How old are your water param tests? Could be giving you false readings. Just a thought.