Hi Alexandria and welcome to the forum. I'm so sorry for your loss. Anhel asked me to give a few tips on lowering ammonia that is already in tap water so here I am.
For tap water that has ammonia, there are a few things you can do to help lower it. One is to use Seachem Prime as your water conditioner, since it detoxifies ammonia by turning it into the less toxic ammonium. Another thing you can do is, if you have a filter, use ammonia chips (also known as zeolite). The zeolite neutralizes ammonia. It depends on your filter if you'll have to use a filter bag in addition to a regular filter cartridge. And finally, floating plants like duckweed, salvinia, and frogbit will help to lower ammonia as well.
If you find you can't get the ammonia level down past .25ppm no matter what you do, you may need to use a half and half mixture of spring water and tap water. You can also use all spring water but some bottled waters have been filtered and refined so that many minerals that fish need have been taken out. If you use all spring water, I'd recommend you get a mineral supplement like Kent's R/O Right to add to it. Don't use distilled water, that definitely has no minerals.
It's possible your guy may have had flukes as well but we probably won't ever know without a skin scraping and a microscope. Ammonia poisoning symptoms can mimic gill fluke symptoms so it's hard to tell. Either way, the advice Anhel gave about cleaning the tank should take care of any parasites that may have been in the tank.