@OmuNegru Just to be clear, when people here are saying the store might have given you waste water, they mean from the waste line of the RO/DI system, they are not saying that it's dirty drain/sewer water.
When you run water through an RO/DI system, part of the water comes out with (hopefully) 0 TDS, and the rest that normally gets wasted down the drain has really high TDS (because it contains all the stuff that was taken out of the "good" water). So if someone at the fish store mixed up the "good" output line with the waste water line, they could be giving you the wrong stuff without realizing it. Definitely should let them know.