I fairly new to fish keeping. I haven't lost any fish til now. I have had 10 tiger barbs for aleast a month and 2 of them seem to be alot smaller then the other. They both often would not swim with the shoal and just stay at the top of the tank by plants or the filter. We 1 of those 2 died i guessing the other might die to but i think he did actually eat this morning. What do i need to do, is it fine to just wait and see or do i need to treat the tank? I don't have anything to test the water except the ph tester and it seems good about 7.4. I just did a water change the other day so i would think all my other levels r ok.

Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

Any signs of flicking and rubbing against objects.
Any excess mucas on the body of the fish.
Check the gills over to see if there pale with excess mucas, or red and inflamed,
Does he look pale or darker in colour.
Does he look thin or bloated.
Have you managed to see him go to the toilet.
Just anything to go on.
Does he hide away.

It's a 29 gallon tank, I have 9 tiger barbs, 4 giant danios, and 1 rubbernose pleco. I don't anything but the ph 7.4and water temp is usually in between 78-80 degrees (sorry i know i need to get the other testers). All the fish look and act normal except the 1 tiger barb, he is small and really thin compared to the others. He does swim around but slower than the other fish and he quite often will just stay at the top

Look really skinny can be old age, fish tb to internal parasites, flukes, velvet.
Can you issolate him.
Signs of internal parasites are.
Being really thin, or bloated.
Long stringy white poo, or clear mucas poo.
Worms putruding from the anus.
Enlarged anus to red inflamed anus.

Once the fish is really skinny they are infested with internal worms which damages the insides of the fish.
The best thing to do is destroy the fish.
If its internal parasites you will have to medicate the whole tank to save the rest.