There is no certainty that the copper is causing the problem, I'm not saying it may not be, just a caution before jumping too far.
The level of copper in Seachem's Flourish [which I believe is kitten's fert, though I may be wrong] is not sufficient to cause any issues with fish, plants or invertebrates, provided the recommended dose (once or twice max per week) is not exceeded.
Plants detoxify copper in trace amounts, just as some water conditioners (those that detoxify heavy metals). In both cases, we are talking trace amounts, the limited amount of copper (or other heavy metals) that would normally possibly be in tap water. Health authorities keep fairly close monitoring if such metals are naturally present in their source water. Of course, copper plumbing as most homes in NA have can be a source of high copper, esp if new. I know of aquarists who have "suddenly" lost all fish and plants due to new copper water pipes. If you get a green residue from the tap running in the bath tub for instance, that is copper.
Most ich remedies are copper based and contain far more copper than fertilizers; which is why they warn you not to use them with invertebrates, plants, and half-strength for sensitive fish (characins, catfish, loaches, etc).
A note on the plants point, plants assimilate copper as a nutrient, but they also "take up" excess copper and detoxify it, as they do excess iron, nickel, manganese, or whatever. [Another benefit of plants in fish tanks.] Again, this is assuming trace amounts; we're not talking polluted toxic water that has discharge from a mine or something, but water that comes out of our taps.