Join Date: Jan 2014
Location: St. Louis Missouri
tl dr: Start dosing the limiting fertilizer.
I was in the same boat Cow, However, I have a 3 gallon tank so cleaning them off wasn't horrible. I have defeated my diatoms. Here is my story.
3 Gallon Spec. 1 Betta. Started in January.
The tank was all good for the first month or so and then diatoms reared their ugly head. I, like you poured over the forums for the nugget of information that would free me from their ugly brown grasp.
I removed all the rocks from the tank (silica source?)
I tried phosguard (remove silica?)
I tried lots of water changes
I added excel each day
I tried amano shrimp
I tried nerites
I upped my photo period
I dropped my photo period
None of it worked.
What finally got me down the right path was a mystery snail dying and another mystery snail dying. For some odd reason they wouldn't flourish. This is an office tank BTW and that makes a difference. My tanks at home have mystery snails and they are happy as um. snails? So I started bringing in water from home.
Now the water at my house is as hard as a rock like 18 drops for both gh and kh hard. So I bought some filtered water from the grocery store. Glacial Water has kiosks around me. For $.50 a gallon I can get RO water. I cut it 50/50 with my home water for something resembling normal water.
Upon adding it to my tank at work my nerite snail perked up and got to town. More importantly the diatoms took a dip. They didn't come back around the edge of my tank like they used to. Sure they were still there but what took 1 hour to clean now took 10 minutes. Answer. Something in my office's municipal water brought on diatoms and killed my snails. Silica? Copper? Who knows? So, you could try a different water supply or even go the RO route but at 90 gallons that may not be feasible.
Part 2 and probably the better advice. About this time my anubias (all 4 of them) and staurogyne repens fully matured in the tank and were adding a new leaf a week. Checking my phosphorus I was always at 0 ppm. My nitrates always hovered around 10 ppm. I decided to start adding phosphorus and potassium to the tank. Now I'm not a fert guy. I'd prefer to let bygones be bygones. However my hand was forced with a few spots of GSA showed up on an anubias.
Dosing up to .5 ppm of phosphorus weekly and my plants exploded. Nitrates dropped to almost 0 (more on that later). New leaves were popping up everywhere. And the diatoms were decimated. The Aha moment was that I had limiting factors in my tank equation. I was stunting my growth via phosphorus and thus my nitrates were burning high. By adding more of the limiting factor I allowed the uptake of nitrates by the plants and removal of the food supply for diatoms.
So now I have a new problem. My nitrates are 0 so some purple algae (bga I think) has just showed up. I have some seachem nitrogen in the mail to fight that. In effect I am doing my own estimated index. Just taking forever to get there.
So in closing. Get the array of seachem products Flourish, Excel, Potassium, Phosphorus, and might as well get iron and nitrogen while you're at it. Bring up the values slowly and you'll starve out the diatoms.
Good luck, you'll get over this hill and there will be another. But the journey is all the fun.