I've been using Seachem Flourish and Excel in my planted tanks with some success but I noticed that the Flourish gets dark over time. I'm assuming that this is due to oxidation of the nutrients inside. If I get the little bottle, I use it up before it gets really dark. So here's my question - is it still good to use? Are the oxidized nutrients still in a form that is available to the plants? I also recently heard or read somewhere that when you buy the liquid plant fertilizers, you're paying for some very expensive water in the process. I'm assuming that the point was being made was that the dry powder is just as good and more economical. Any opinions? What are you all using?