My TDS was 8ppm and nearly killed my tank with copper a number of years ago. Apparently with such a low TDS it "scavenges" more copper from the pipes than higher TDS water does. I tested my tapwater with a copper test kit and was amazed. Killed a number of inverts. I've been using RO/DI ever since. And still, even with such a low TDS, the first sediment filter gets dirty and the carbon is used up on chlorine. The RO and DI lasts me a long time though. Another thing to consider is you don't know what comprises of that 26ppm. You only know it's dissolved solids that are safe for human consumption, but that's about it. So to answer your question, better be safe than sorry, use RO/DI.
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