We have a 20 gallon-long tank (long established, fully cycled, recently tested all specs normal) with 4 Blackskirt Tetra, 3 Julii Corycats, and one Nerite snail. Last week we added 3 female swordtails. They started eating the algae on our plants like crazy which gave them black stringly poop. The store told us this was normal. After a few days one of them changed to white stringy poop. It started to clench its fins, hang out first at the top, then at the bottom, and died a day later. The other two were fine. The store told us it was probably transportation stress and gave us a new one. The store also mentioned it could be parasites, but none of the local pet stores had the tetra parasite guard in stock so we ordered it. Should come in tomorrow. I just saw that two of the swordtails (one of the old ones and the new one) are showing the same behavior like the one that died. All the other fish are acting normal. What is happening - are these parasites or could they be overeating on the algae? Please help! Clearly it's something in our tank. The new one had been at the store since last week (recognizable because it is missing a small piece of fin) and was doing just fine there.