Sorry to hear about your fish. In order to help, we're going to need a lot more info. Answer as many of these questions as you can please.
1. Size of tank?
2. Water parameters
d. pH, KH and GH?
e. Test kit?
4. FW (fresh water) or BW (brackish)?
5. How long the aquarium has been set up?
6. What fish do you have? How many are in your tank? How big are they? How long have you had them?
7. Were the fish placed under quarantine period (minus the first batch from the point wherein the tank is ready to accommodate the inhabitants)?
8. a. Any live plants? Fake plants?
b. Sand, gravel, barebottom?
c. Rocks, woods, fancy decors? Any hollow decors?
9. a. Filtration?
10. a. Lighting schedule? What lights are used?
b. Any sunlight exposure? How long?
11. a. Water change schedule?
b. Volume of water changed?
c. Well water, tap water, RO water?
d. Water conditioner used?
e. Frequency of gravel/sand (if any) vacuumed?
How often are they fed?
13. a. Any abnormal signs/symptoms?
b. Appearance of poop?
c. Appearance of gills?
14. a. Have you treated your fish ahead of diagnosis?
b. What meds were used?
15. Insert photos of fish in question and full tank shot if necessary.