this is a quote taken from a buddy at another forum about 5 minutes ago.....regarding a plant that many are having trouble growing...
AS will drop the KH to zero if it's not above 10 or so out of the tap, with nothing in the tank to raise it. I've noticed that plants not adapted to very soft water will die off quickly when exposed to AS, but they will grow back with patience. It's just their adapting to a new enviornment. Fish can take it hard at first, but they really just need to adapt. Go easy on the CO2 at first. Observe and adjust.
After about a 2 months or less, the KH will begin to rise naturally for some strange reason, up to about 3-4 dKH. It doesn't really effect the plants much, but you will need to up the CO2 some. However, plant growth is typically still very lush, especially if you key into the KH rise.
Frankly, I don't care what the GH is, and I'd prefer the KH to be at zero, but for some reason it rises a bit. Raising the KH will effectively kill Tonina's, but if they have an acidic substrate, they will hang on a bit longer. I don't care as long as the plants are growing--when they stop, which is rare, I investigate. So far, I've only found inferior substrates to actually cause a growth problem. There's a bunch of nonsense that low KH's will cause 'dramatic' pH swings, but that's a bunch of, well, nonsense! Looks good on paper, but doesn't pan out in the real world.
My 'acidic' tanks have operated at super low pH's, KH's, GH's, and I've never had better results! The only warning I will provide is watch the CO2. It's the only thing that will kill fish. Be ready to adjust as necessary.
If you have good water movement you are all set...no fishy problems.