TDS >350 Substrate Mistake(s)?
I live in Austin, TX, and our tap water is hard, TDS about 150 according to my simple dip meter. I've had my 90-gallon tank filled for about five days and I've been slowly siphoning out water and replacing it with RO/DI water (all new filter cartridges) that measures about 15 on the same meter. I've replaced maybe 15% so far. I have some swords and a few other plants planted with Seachem fert tabs and a little Excel and Stabilizer and so forth. Nothing exotic, everything conservatively dosed, for example the Excel is limited to 1/5 of the directed dosage.
I've been waiting for the nitrogen cycle to do its thing, but ammonia is remaining at 0.5 mg/L, with about 12 mosquitofish enjoying their new digs after being in a tiny "pond." For a while I had a bunch of pond plants floating in the tank, so I suspect/hope my tank is more cycled than I had first suspected. A nitrite and nitrate test is in the mail.
I hadn't bothered to check the tank water for hardness until last night when I was absolutely gobsmacked by a 350 reading! WTH? Today I will begin a major water change, perhaps >75% over the next two days. But now I need some knowledge and guidance. The substrate is thin and a little haphazard: two 7kg bags of Seachem Flourite, maybe 30 pounds of pea gravel, 40 pounds of playground sand, and two medium-sized rocks that I believe are not sedimentary in nature. I also have about 40 pounds of so-called "egg rock" from Lowes. I have another 14 kg of Flourite coming in later today. Which of these is most likely the water-hardening culprit?
Thanks for your help and expertise!