Is this a newly set up tank, Phil?
Welcome to the forum, btw
I expect your problem is that your tank isn't 'cycled'. That means it doesn't have a colony of good bacteria living in the filter. The bacteria eat the ammonia produced by the fish and turn it into less toxic substances; first nitrite, which is also toxic, and then nitrate, which is only toxic at very high levels, and which we keep low with water changes, in a cycled tank.
Try and change at least 95% of the water right now; leave just enough water for the fish to swim upright (don't forget to switch your heater/filter off first!) before refiling with warmed, dechlorinated water. Then do 50 or 60% every day until you can afford some test kits to monitor the levels of those toxins (ammonia and nitrite). You don't need to worry about pH or nitrate at the moment.
Hope that's useful; if you could post more details of your tank (size, how many fish and what kinds, temperature, filtration, plants) that would help a lot too.
