serious aeration issue - need advice
I have 2 27 gallon hex tanks. I have long kept my substrate as a mix of small particle fluorite and inert black sand. I have noticed when vacuuming them that there is a foul odor. I have also noticed issues with plant growth and anaerobic bacteria in one of the two tanks. The other has more plant life than any tank should (lightless jungle by the time you hit substrate level), and no real issues.
I am considering, and after a fairly long search managed to locate an undergravel filter for the tank with issues. My thought is to remove all desirable life, take a hose and suck out the sand. Then to put the undergravel filter in place, rinse the substrate and reinstall it. I will keep my canister filter for real filtration, and my UV will run during the process to help kill off anaerobes (though the oxygen will likely take care of them).
Here is my concern, the chemical which causes the odor (whose name I can't recall) will likely be present in the sand that is returned even after a good rinse, and this has already proven to be mildly toxic to the tank's crystal shrimp inhabitants.
I am debating hooking the intake to the canister filter up to the undergravel at first in hopes that the purigen will help pick this up. I don't usually run carbon, but can if it will help. Any thoughts? I don't want to kill the crystal shrimp or ottos that inhabit my tank, but it obviously needs help. Also if anyone knows where I can find a second filter (they seem to have stopped making them), please let me know. If this works out well I will do the other tank just to prevent issues. Substrate depth in the tank with issues is about 3", the other is about 2".