Hi again
Right, they are both definitely fancy goldfish - the healthy one is a fantail (and he does look healthy to me & not fat!
), can't really see the poorly one very well in that pic but if he's the same shape as the other then he's probably a fantail too.
Brace yourself coz here's the bad news - I make that tank out to be roughly 20 litres which is much, much too small for a pair of goldfish. It's generally recommended that fancy goldfish have at least 40 litres
each, which means you'd need at least an 80 litre tank for the pair of them (preferably more).
I know that sounds like a hell of a lot but there are good reasons for it. Your daughters goldfish are only babies at the moment, but they should reach an adult size of around 6 - 8 inches (not including tail!), and potentially live for around 20 years. The sheer size that they can grow to means that they need a lot of space to exercise. Goldfish also produce an
awful lot of waste (they're one of the messiest fish you can get!), which means that the water gets very dirty very quickly.
Unfortunately, as well as selling you the wrong kind of tank, it doesn't sound like the people at the fish shop told you anything about cycling your tank. In a nutshell the purpose of cycling a tank is to mature the filter (by building up good bacteria inside it) so that it can cope with the waste the fish produce and make the water safe for them. I'm guessing that your filter is not coping and your water quality is suffering because of it. Essentially, if the filter is not coping then the fish are effectively swimming in their own waste, which is very toxic to them and will be the reason your fish is so unwell
Do you have a dechlorinator (stuff that removes chlorine & chloramine from tap water)? If yes, then the best thing you can do is a big water change (75% or so), making sure you dechlorinate the water first and try to get the temperature of the fresh water the same as the tanks current temp. I would continue to do this on a daily basis (changing maybe 50% of the water every day) at least until you get a test kit and can see what the water is like, because it won't take long for the water to get dirty again.
Then you need to get a test kit. NOT the ones that are strips of paper that you dip in the water coz they're rubbish(!) - you want a liquid drop test like the API Freshwater Master Test Kit or the Nutrafin one (most fish and pet shops sell these brands). They cost about £20 but it'll last you for absolutely ages - you get hundreds of tests in it.
Lastly, those fish are really going to need a bigger tank and as soon as possible, with a big filter that is suitable for a tank twice the size as the one you buy (because golfish are so messy, a big filter is vital to keep the water clean).
I'm sorry if that all comes as a bit of a bombshell. Unfortunately you've been given some truly terrible advice by your fish shop, which is depressingly common. Hopefully we'll be able to save your daughters poorly goldfish though
Edited to add: I would also feed the fish sparingly for a while, as more food = more waste and you'll have a harder job keeping on top of water quality. If the poorly fish is that unwell then he's not going to eat no matter how much food you put in there, and it won't do the healthy one any harm to be fed a little less than usual.