You will need to test for ammonia and nitrItes, as well as nitrAtes. ammonia and nitrItes, even in small ammounts, can and are deadly to fish - regardless of species.
I would also advise against trying to lower your pH as the fluctuations are more dangerous than the constant high pH itself. Guppies, mollies and other livebearers don't do too bad in a high pH anyway and 7.4 is not that high so you can leave this alone.
I agree that you should try feeding the fish a shelled pea and see if what it has is swim bladder disorder.
How long ago did you take the bio-wheel out? This contained a lot of beneficial bacteria and taking it out could be very detrimental to your biological filtration and, thus, water quality. Testing your water is the only way to ensure it is safe for your fish.
Also keep in mind that adding 5 fish to a small, 15 gallon tank can result in a temporary ammonia and nitrIte spike that can be quite dangerous for your fish. Adding fewer at a time will help.
Quarantining, as mentioned, in a seperate tank for at least 2 weeks can be very beneficial and means you can treat any sick fish without worrying about the spread of disease or the effect medications will have on your biological filter.
I would start by doing a large water change to correct any problems with water quality. Make sure you are using de-chlorinated water and don't try to control pH. Then try feeding the shelled pea and buy a test kit and post your results for ammonia/nitrIte and nitrAte.
Good luck
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