Hmmm clean it in tap water?
Personally, i think that in having the water that low, you are going to be a bit over stocked because there isnt that much water for that many fish if water is so low. I would definately advise getting a new filter, something like the fluval mini if not the U1, they arent the most sightly when you are used to undergravel filters but i think when it comes down to it, even if it isnt the cause of your problems now (which i think it will be...) i would reckon that your water quality is quite poor and whilst these fish are tolerant enough to survive it, it will have severly lowered their immune system to a point where they all pick up different problems easily.
I would think of it as very urgent and see if your nearest fish shop, maidenhead aquatics and pets at home all do water tests, get them to test your water, get them to write down both the name of the test kit (eg i use Interpet EasyTest) and your results for Ammonia, Nitrite and Nitrate. Then you can share with us your test results and we might be able to help you further!
I would get this done asap this weekend!