Back to Basics (but better!)

Unfortunately Garnet died this week and I'm not too sure why. I'm also noticing yellow leaves on some of the plants but again I'm not too sure as to why that would be happening. The pH of the nutrients is down at 5.7-5.4 within that range so it could do with coming up a tiny bit but I'm really not convinced that such a small offset in pH could cause that much damage to the plants. If anyone has any ideas on what to check or do, feel absolutely free to comment away any ideas that you think might be happening here.