Help! Fish Opening Mouth/not Eating

I have 4 green neons, 2 black tetras. 25 gallon tank.
I have and air stone! So I doubt it's a problem with oxygen.

I got the black tetras 4 days ago, they were very active, eating, and all. I added the 4 green neons 2 days ago and they were very active as well

My tank is 4 months old, been running the whole time, I tried adding fish (6 fancy guppies) 2 months ago but they stayed at the top of the tank and after a few days died. I let my water cycle for 2 months, after doing several water changes.

I've tested my water, and all the perameters check out, I'm worried about my fish, they keep opening there mouth maybe 2-3 times a second, they stoped eating as well. They stay at the back of the tank at the bottom. But they still move around alittle, what could I try?

 

What are the test readings on the Ammonia, Nitrite and Nitrate?

 

It's 0 ammonia 0 nitrite almost no nitrate I can't remember the reading, but it was almost at 0 my ph is 8, same as my tap water

 

There is nothing visually wrong with the fish? Color, bloating, spots or patches?

 

Not that I can tell! They don't seem different from when I got them besides that they open there mouths way too often, the green neons seem to have lost alittle bit of colour I suppose, alittle less brilliant... but besides that I don't notice anything

 

I've been doing daily water changes of 25% since this started, I'm using water conditioner too, could they possibly all be sick?

 

Anyone?? I've done a 70% water change and I removed half my substrate, really not sure what to do here, should I try and medicate?

 

Maybe you can try a 100% water change, and cycle the tank again? This could be a virus in the water or infection, this probably isn't the case but you could try. this could also be because of maybe something in your tank (a decoration that is harmful, or a plant). I also recommend getting some floating pellats if this continues.

 

Floating pellets?

 

Floating pellets?

Sorry! I meant sinking food

 

Ah yes, I'm going to go get some, I'm also going to get some medication for them

 

Have you noticed any flashing?