tiger and albino barbs

Question
i just changed the water in the aquarium about 3 days ago  i have 6 albino barbs and 6 tiger barbs;  and i just noticed yesterday that one of the albino barbs didnt want to eat it was just swimming in the same place for a while.  Then i just noticed this morning that it was floating ontop of the water.  Then when i came home today 3 of the tiger barbs were at the bottom of the aquarium.  Please tell me why are they dying and what can be done to prevent this....thank you!

Answer
This is very strange. Are you sure you added the declorinator when you replaced the water? I would first suspect that, as I have done this a few times myself. Next, are there any signs of a disease or parasite such as: bumps, fuzzy spots, spots like salt, darting on gravel or decor, velvet like spots, bloated bellies, torn fins, heavy breathing, not eating at all, or any other odd symptoms you have not had before? Until I know this it is nearly impossible for me to answer the question. You might also want to test your water, as maybe something is really off, like the nitrate, nitrite, ammonia, or pH. Other than testing the water and making sure you added declorinator I will need to know more information. Without any symptoms it could simply be from stress. However, even if there are no odd symptoms, it will be nearly impossible or me to give a good answer as I nor you would have anything to go from. Please let me know any odd symptoms the fish have and I will try ti help you more.