Sick gardneri killifish?

[koaladarshana]

One of my three gardneris was looking quite lethargic the day before yesterday so I slowly added 6 teaspoons of aquarium salt yesterday throughout the day (it's a 10 gallon tank).

Today he is looking a lot better, however, another gardneri is now looking very lethargic. It is just lying on the gravel at the bottom. It is not even trying to swim unless I try "spooking" it (I know this just stresses them out but I had to make sure it was still alive).

The three share a tank by themselves where I change 50% of the water once a week and siphon the gravel just as often.

My readings are as follows:

Nitrate: <10
Nitrite: 0
Ammonia: 0
PH: 7.2

Water temp. is 76 degrees Farenheit.

Right now my instinct is to do a large water change to remove the salt but then again I'm not completely sure that is the problem as one of the killies looked sick even before I added the salt...

What should I do?

[koaladarshana]

I forgot to add they are fed a rotation of spirulina flake, micro pellet (they are only juvies about an inch and a half in length) and frozen bloodworms.

[koaladarshana]

P.S. Also forgot to add that they are two females and one male, and I haven't noticed any bullying whatsoever.

[koaladarshana]

An update: Another gardneri has fallen ill (the one who was sick the day before yesterday). That's two now that are just laying at the bottom not even trying to swim. I did a quick 50% water change as a precaution with an added 1.5 teaspoons aquarium salt instead of 3. IDK what I'm doing at this point...