Every plant in my tank is thriving and growing like a weed, except for in one area I like to call the "dead zone". It's approx. a 4"x4" area that sits about 3 inches back from the front of the tank and slightly off to the right side.
There is good flow over the "dead" zone and light is medium/high and unobstructed. Tank is CO2 injected dosed at 30ppm and off at night. PH fluctuates between 6.8 and 7.6 based on when CO2 is being injected. I dose Macros and Micros at slightly below EI levels. Lights are on for 8 hours a day. Tank temp is 78 degrees. When I set up the tank 4 months ago I pushed ~20 gelcaps filled with Osmocote Plus into the sand substrate. For the first month or so I would poke around in the sand with my tweezers to release any gases that had built up (there were no fish in the tank at that time).
Tank has been setup for 4 months and heavily planted for 3. The first 2 months all plants were doing great. Month 3 I saw a good bit of diatoms and hair algae. Tried the one two punch method to kill the hair algea, and that's when I started noticing the "dead zone". I brushed it off and told myself the plants in that area were probably just not happy with dosing the tank with H2O2 and excel, but they'd bounce back in a week or so. Welllllll, that has not been the case. The plants in the "dead zone" are getting worse as time goes on.
Plants I'm trying to grow in the "dz":
marsilea minuta and echinodoru tenellus (pygmy chain sword). The marsilea minuta outside of the "dz" are flourishing and spreading like crazy. Any that dare venture into the "dz" quickly turn yellow/black and die. Unfortunately, the pygmy chain swords are planted right smack in the middle of the "dz" and the leaves are turning yellow and transparent.
What is causing a small area in my tank to become toxic to my plants? Is it possible that I put too much Osmocote Plus in that area and the plants are being burned by too much fertilizer? That's the only explanation I can come up with.
I'll take a some pics of the area in question and post later tonight.