Dying Fish

I would also ask if you have those figures quite right. The lowest ammonia and nitrite reading with the API tester is 0.25ppm, you can't measure between that and zero. Are the readings actually 0.5?
And the lowest nitrate over zero that can be measured is 5ppm; should the nitrate reading be 10ppm?

I didn't do this reading myself as the API tester only arrived today, I had these done at the Aquatic centre.  As for cleaning my filter, as it's a new one I'll just leave it for now but with my old sponge filter, I rinsed it in the tank water I removed and then added either a gel or Pure pearls just to give the water an added bit of help. The Aquatic centre have reassured me that the water is about as good as you can get it and considering the filter is only a couple of weeks old we were all pleased.  The tank however, has been functioning for 18 months

If it was the shop that gave you those figures, it wouldn't surprise me if they got them wrong. The safest thing is for you to use your new tester and check the levels yourself. Ammonia and nitrite are the important ones as nitrate testers are not terribly accurate anyway. It needs a lab set-up for accurate nitrate.

Two things to be aware of.
Fluorescent lights (including compact fluorescent energy saving bulbs) can make the liquid in the tube look greener than it really is. If you still have any old fashioned, incandescent light bulbs, use that for reading the ammonia test.
The instructions for the nitrate test include shaking bottle #2 before using it, and shaking the tube well after adding the drops. These shakings are important. One of the reagents in bottle #2 settles out on the bottom of the bottle - all that shaking is to redissolve it. This also applies to other brands besides API.

i wish there was a way to give a thumbs up on here sue that is excellent
advice you taken the words out of my mouth great minds thing alike

The kit they have looked massive and complicated but then it would to me lol.  They only do fish, marine, tropical and ponds.  I've been testing every day with the strips as that is all I had but will test with the API tomorrow and let you know how I go 

Keep an eye on it, don't let the ammonia get much higher than that.

Altering pH is not something to be undertaken lightly. Because your water is hard it is likely to have a lot of carbonate (KH) as well as calcium and magnesium (GH) and carbonate buffers the water against pH changes. This means that if you add a pH down type of chemical, the water will resist it and the pH will drop initially then rise again, and this yo-yoing pH is bad for the fish.
You could try putting a lot of bogwood in the tank, but if the water is really hard that won't have much impact, I'm afraid.

Hardness is more important than pH to fish. Do you know how hard your water is? Your water supplier's website should have it somewhere. It is much better to have fish that are happy in your water rather than try to alter the water to suit a fish species.
The first thing to do is find out the hardness, making note of which units the site uses - there are several they could use, a bit like feet and inches versus metres and cm. Then look at the fish you have/would like on Seriously Fish for their preferred hardness and pH, and also minimum tank size. Finally, enter your tank and fish in the community creator on here to see whether the fish would be OK together, and how stocked the tank would be. (Have you found the CC yet?)