Congrats on the purchase. I'm on the West Coast, and usually receive my orders in about a week.
As for the tap water, you should be able to get just about any fish. A stable
ph is better than trying to change it. I use driftwood in my tank to bring the
ph down a little naturally. That way it remains stable. don't use chemicals to adjust
ph, they are only temporary, and
ph fluctuations are hard on fish.
As for the stringy poo, I wouldn't think a parasite. When all my fish poo, it's stringy. The only time you should be concerned is if it starts looking different, like a different color as in white, etc. My tiger barbs always have stringy poo, lol.
As for the dead fish at the store, that would be a concern. And because there's more than 1 tank leads me to believe the tanks filtration are probably the same system, so if a problem occurs in 1 tank, will spread to the others. Our
lfs here has a system for every 2 tanks, and each tank is divided into 3 compartments. So the same filtration system works on 6 compartments. When looking for the fish, look at all the fish in that tank, as well as nearby tanks that might be on the same system.
As for flake food, it's good for most fish. You can get it, then when you select your fish, you can get other foods to suppliment the flake food. It's good to have 2 or 3 different items to give a variety, as well as fresh vegies like zucchini, cucumber, and peas on occasion. I usually do flake food, then sometimes add some freeze-dried bloodworms or brine shrimp or plankton if the fish is big enough. I also have frozen bloodworms and frozen brine shrimp as well. But remember, don't overfeed.....