It would be good to have other opinions here, but I would recommend a Phosphate test kit so you know what you are dealing with, I would recommend the Seachem kit very easy to ready and quick to use. My guess is, IMO, that you will find your PO4 (Phosphates) high. The question is where is the problem. With TDS at 15 maybe your water source. Someone else may have a better idea on a TDS of 15 being phosphate. Add a DI and you should be down to 0. But if I am not mistaken that is one the main reasons to use a DI filter, phosphates. They aren't very expensive to add, see the filter guys advertised here on RC. If your tank is the culprit, like mine, which is my best guess (maybe both), then there are a bunch of ideas presented above. Ryan also told me about dosing sugar and I think it is a feasible idea. But first I would read the posts Ryan pointed me to. Search on RC for "sugar nitrates 101" The post is huge, but worth the read. There is some dissension about dosing sugar. Mainly because there is no scientific proof that it works, so why take the chance. MANY swear by it and say it is widely used in Europe.