It could be a few things, but generally its not a good sign.
Most importantly... test your water (or get a shop to do it and write down the results)...
You need to know:
Ammonia
Nitrite
Nitrate
pH
Poor water quality could come about in any number of ways and not be visable to the human eye but would cause 'burning' and discomfort to the fish and cause them to 'flick'.
Also, and just as commonly, it can be a sign of external parasites making the fish really itchy and sore.
Obviously, adding a treatment for water quality wont help if its parasites and adding a parasite treatment can make situation worse it its water quality problem...
So very important to test that water asap!