[Taras1]

When was your last water change? It sounds like your buffering capacity has been overtaxed. You might have a build-up of organic acids which counter the inorganic Ca/BiCarb/Carbonate buffering. Check your skimmer. Also if you have any filtering material, bioballs, etc. make sure they're clean or changed out. Sometimes a brief (a day or so) filtering with carbon can help.

I've used Red Sea and not had problems with it. Personally, I use Reef Crystals right now.