I've been trying here in Durham NC for several months to grow lots of diatoms for my hillstream loaches; in Oregon they spawned regularly in a 20L w/lotsa rocks & current, & i almost daily traded out diatom-cevered rocks from my big E. canarensis tanks --those hillstreams can eat an astounding amount of diatoms. The water in Or was soft (GH & KH ~2-3, neutral pH, I added a little potassium to the cana tanks for the java moss & bolbitis.
Here in Durham the water parameters seem to be about the same, but I'm going bananas trying to get enough diatoms for the hillstreams. I did some research on diatoms & found the main requirements for their growth seem to be phosphorus, silica, & pectin (oddly enough). As mentioned before, silica is nearly insoluble (except in ....I think it was hydrofluoric or maybe picric acid....not what you want in your tanks). I've tried supplementing with varying amounts of phosphorus & pectin & some pure silica gel from my lab, in tanks & buckets & tubs, high, low, & medium light, etc etc ad nauseum. The only definitive conclusion I've reached is that it's impossible to grow diatoms if you really want them, & impossible not to if you don't want 'em.
My next experiment is using diatomaceous earth along with the phosphorus & pectin. I'm going to try adding a mild acid (maybe boil some in white vinegar, since it will boil away), maybe grind some up....the blasted silica that formed all those multi-gajillions of diatoms that make up say, the cliffs of Dover & all the other diatomaceous formations all over the earth must've come from somewhere, & I'm determined to get some silica out of those frigging dead diatoms somehow.
I hope. As Scotty said, I'll let ya knoo.
Oh, I tried the ground food in the egg-white trick; the hillstreams didn't much go for it, the 2 otos in the tank ate a little bit, but oddly enough the furcata rainbows were most interested in it. I did find the food floating off the rock after about 5 minutes, tho; perhaps I didn't let it dry long enough? I left it out for about 2 hours before putting it in the tank. My Etroplus canarensis, of course, went nuts over it. They eat awfuchs in the wild, & in the 6 years I've been breeding them I have yet to find anything they won't devour like starving wolverines, so they're not exactly helpful critics.