Well, I am at a loss as to what is causing the water to stagnate in my fish-only tank. I have a 400 GPH Marineland HOB filter and two 400 GPH powerheads; still the water becomes stagnant..... shouldn't be any dead spots in the tank; the substrate (crushed coral is only like and inch deep or less.... Now making several 25% water changes to try and combat the stagnation; and adding a 300 GPH HOB filter.

