tropical fish tank

Question
hi, i have recently invested in a tropical fish tank. it is a 25 litre tank, it has a air pump, a good heater and filter and a mixture of real and fake plants, and some decorative rocks.  i broke the tank in with a shoal of 6 tiger barbs which have got along great, a couple of days ago i introduced a small angel fish and 6 neon tetras. i change 20% of the water weekly, use a solution designed to level out the ph, carbon, nitrite and nitrate within the water.  since introducing the new fish to the tank. one of the tiger barbs has been swimming very close to the surface and at times seems to be breathing very rapidly. the angel fish also seems to be spending a majority of its time at the surface? they are not over fed and the water is clear, i vacum the gravel every few days.
is there anything you can think of as to why they are doing this, they dont appear to be suffering any illness and all the rest of the fish are fine.
thank you for your help
helen

Answer
Hi Helen,
I'm sorry to say, but your Angel Fish is out of place in a 6 gallon tank.  An Angel Fish needs 20 gallons of water in order to be happy, and healthy.   It also isn't compatible with Tiger Barbs.  A 6 gallon tank is too small to house all these fish.  When introducing the Angel, and the neons, your ammonia level went up, and this can be deadly for fish.  Products to level the PH is not good, as your PH will fluctuate, and this too is very bad for fish.  The only way to get rid of ammonia and nitrates is through water changes, and not over stock your tank.  You will have to bring your Angel, and tetra fish back to the Pet Store, and ask for a credit, or your Angel fish, and other fish will die.   Your tank also lacks oxygen for all of these fish which is why they are swimming near the surface.    
Bring your fish back, ask for a credit, then do 10% water changes every day until the ammonia in your water is gone.
I wish I could have given you better news, but this is the situation right now, and there is no other way to help you with your fish.
Lynda