Re: I need a good aauarium dhop in london to help Seth
I can't help with London shops as I'm a long way from there, but if you have a 55-gallon tank it's plenty big enough. Did you maybe mean 55 litres? If so you will need to upgrade to at least 180 litres for your goldfish. Have a look at these articles:
What is the kH of your tap water? It may be low, but possibly enough if you up your water changes to 25% weekly or a tad more rather than adding a buffer. That way your pH won't rise as much. Though to be honest, the pH is unlikely to be an issue if your water is soft, which is the more important factor for soft water fish.
As to the disease, that's a separate problem, and I would suggest treating with Esha 2000 to try and cure that. What water conditioner do you use? Some can actually make medications less effective so that may be why treatment has been less than effective so far.
Guppies are a hard water fish. In soft water they can suffer the kind of problems you're seeing and eventually die. The problems related to soft water are long term and cumulative so take time to actually manifest.
Your nitrate is a little high (depends partly on how much is in your tap water though) and could be the cause of the licking. Otherwise it could just be a bit of sand causing irritation. Ideally these cories need a tank a bit bigger than yours - 2ft long/60 litres and to be in a larger group of six or more, but that's not practical in a 40 litre.
Sorry to hear they've deteriorated so fast. That does indicate that it's likely to be a congenital deformity. You've done your best for them so although this is very sad, there really wasn't anything more you could have done.