Well I think this topic should be moved a bit into the beginner help forum. I hate to be the bearer of bad news but I'll tell you a few problems I see.
Firstly for being a only a 7.5 gal your tank is overstocked what you would like to look at is the adult sizes that fish grow. Not to mention a lot of what you have in your tank are concerned shoaling fish which need at least 5 other of the same species in their tank. Tiger barbs need to be in a grouped of 6 as they are a very aggressive species as you have them. You've mostly likely noticed some fin nipping. A group of six tiger barbs need at least 15gal of water just to themselves.
Secondly I've never heard of "blue tail tetras" nor could I find such a species in any of my books. Is this the name that the local fish store gave you? See if you can call them tomorrow and give you the scientific name but most likely they are another shoaling fish.
A 7.5 gal is really not a big enough tank to properly support any of the fish you currently have comfortable to their needs.
How old was the tank before you put fish into the water? Your fish may be suffering the effects of what is referred to as the cycle (information is stickied in other area of this forum)
I'd recommend returning your tiger barbs at the very least and if you would like to continue keeping the fish you already own look at purchasing a nice 20 gal tank (if you don't have the money look to buy used in the buy and sell in your local area).
I'm not sure who instructed you to replace 80% of the water this is going to restart whatever cycle you had going and cleaning your filter and gravel will kill off the beneficial bacteria you've already started to accumulate in your tank. At the very most I'd change 10% of the water a day and only if I had an ammonia spike.
As for you fish gasping try and purchase an air stone and air pump from your local Wal-Mart (cheapest) or LFS the surface agitation will increase the amount of oxygen in the water.
If it is possible could you please post your water parameters?
Best of luck currently and hope to hear from you soon
By the way
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