Dying Fish

Last night, one of my scarlet platies zoomed manically around the tank and then died!  I think it was one of the 5 I bought Wednesday.  I also lost another earlier yesterday, found it lying on the bottom  I wondered
 what could be the problem

They were bought at the same time (wed) same shop/tank.  My first thought was I had brought an infection with them. all the others are ok.  no visible marks

I was going to exchange my tiger barbs for some more scarlet platies, but now i'm not sure.l

I tested water and I think I need to do a water change?.  ammonia 0. Nitrite & nitrate 2.5 would this increase be enough to cause a problem?

I agree. Any reading for ammonia and/or nitrite above zero means you need to do a water change. If the platies had been weaker ones, the nitrite reading would have been enough to harm them.

I would still take the tiger barbs back but ask for a credit note. Then if the remaining fish are OK for the next week or so you can go back and spend the credit note, even if you use it on something other than fish and get fish from a different shop.

Pets at Home varies wildly. They've got a poor reputation on the whole, for being somewhere that doesn't look after fish too well and it's not entirely undeserved. It does vary from shop to shop, however, and some are better than others in this regard. Their staff are also trained to make sales, rather than to help customers with sensible decisions, so go in armed with your own information instead of listening to theirs.

My local P@H has been a mixed bag. I bought some Amano Shrimp from them which thrived in my tank, when I'd had problems with shrimp from a better-rated independent LFS dying off, but I also had a pair of red Honey Gouramis that were both dead within a month of purchase.

I'm nowhere near Yeovil, so I can't advise on if yours is any good. Google informs me that there's a place called Watermarque in Yeovil too, and from threads on here, they seem to be fairly well-rated.

My nearest P@H has an awful selection of fish 

There is not a branch of Maidenhead Aquatics in Yeovil; the nearest are Wellington near Taunton (in Blackdown garden centre), Galton branch (in the Wyevale garden centre at Owermoigne, Dorchester) and Shepton Mallet (in Dobbies garden centre). MA's are a franchise business like P@H so they vary in staff knowledge and management practices, but most are better than P@H. If you can manage to get to an MA, again go armed with your own information.

My nearest P@H has an awful selection of fish 

Galton branch (in the Wyevale garden centre at Owermoigne, Dorchester) and Shepton Mallet (in Dobbies garden centre). MA's are a franchise business like P@H so they vary in staff knowledge and management practices, but most are better than P@H. If you can manage to get to an MA, again go armed with your own information.

Funnily enough, it was Dobbies Shepton Mallet that re-kindled my desire for Tropical fish, and like topsy, it just grew