Hello,
Sorry about all your problems... but please stick with it, this is a great hobby!
First off, Aquarium Doctor WSP is a cure for a specific problem called "white spot", often referred to by Americans as Ick. As the name suggests, this disease consists of small (icing sugar sized) grains all over the fish. Often you spot one or two on the fins first, then some more a few days later. It's pretty common, and aquarists of all skill levels have to deal with it.
Anyway, Aquarium Doctor WSP won't fix other problems like fungus, bacterial infections, poor water quality, old age, or anything else. So for the store to blindly sell you the stuff without checking to see if your fish actually had a case of white spot was very wrong.
Fish swimming weirdly is rarely a problem with white spot. More likely poisoning, bacterial infections, dropsy, and so on. Sometimes it comes and goes by itself, but more often with small fish especially once they get to the stage of swimming upside down, that's the end of the road.
Almost always, problems with sick fish are problems with the tank. This doesn't neccessarily mean you've done something silly but that you haven't chosen the right fish for your tank. For example, pencilfish are delicate, need soft and acidic water, and have no tolerance for nitrite. In London tap water, which is very hard and alkaline, they are basically being kept in the wrong kind of water, and it doesn't take much to kill them. Likewise neons, dwarf gouramis, and a whole bunch of other soft water fish widely sold without anyone telling the purchaser what they need.
If you go to a garden centre and look at the plants there, there will be labels saying that they need clay soils or acid soils or whatever. Likewise fish often have very specific needs as far as water goes.
Similarly, a new tank (i.e., one less than 3-4 months old) is a hostile environment as far as fish go. As someone in TFH magazine once said, a fish swims around in its toilet, but we're the ones who flush it for them. We need to create a filter that removes the short-term toxins (nitrite and ammonium) and make water changes to dilute the long-term toxins (nitrate and dissolve organics). Neither of these things is difficult, but they are subtle.
Too many water changes early on slows down filter maturation, so for the first month or two you want to only make small water changes (I'd recommend 10% a week) and only use a few, nitrite-resistant species. In London tap water, black mollies would be ideal. Alternatively you could just dump a bit (a 1 cm cube) of lean steak or fish in there and let it rot for a few weeks before adding any fish at all. That would have the same effect. You can also add various potions to speed up the process; they work for some people, but others don't consider them up to much. Personally, I find them helpful.
After a month of just a few black mollies you can then slowly build up the community of fish. Use a nitrite test kit to make sure the nitrites aren't too high. Follow the instructions on the box about how often and when to carry them out. These test kits cost about 5 to 10 pounds depending on the brand. I use the dip strips, which among other things can be sliced down the middle to make two tests per strip. Very cost effective!
In London tap water, bear in mind your most successful fish are going to be ones pre-adapted to a pH of 7.5 to 8 and a very high hardness level. Nitrates are also high; here in Hertfordshire my supply has up to 100 mg/l out of the tap even before I add the fish!
I'd avoid gouramis, rasboras, tetras, and South American cichlids (like angels). Instead opt for things like swordtails, platies, guppies, mollies, rainbowfish, gobies, and halfbeaks. Central American and African Rift Valley cichlids also do well in London water, as will brackish water fish provided you add some salt (e.g., monos, scats, puffers, Colombian sharks). Killifish are hard to find in London but some species, such as Florida Flags, do well in London tap water. Hard, alkaline water has several advantages for fish keeping once get past the shorter list of suitable species. It is self-buffering, moderating the pH swings that can be harmful, for example. You can also add sea shells without worrying about what it will do to the pH, and things like snails and shrimps only thrive in alkaline, hard water. So, London tap water is excellent for keeping fish, provided you know which fish to get...
There are also lots of plants that can use calcium carbonate (the mineral salt that makes the water hard) and so do well in hard, alkaline water. These include Vallisneria, Elodea, Ceratophyllum, and Java Fern.
The best you can do with your tank right now is probably to sit on your hands! Adding stuff like potions and tablets in the hope they'll fix the problem isn't wise. Getting an aquarium book from the library should be your first thing to do, and use the health guide in there to see if you can identify specific diseases (if any). Then do a nitrite test. (You could do ammonium, but its connected to the nitrite so you may as well do one or the other, rather than both.) You can assume the nitrates are high because you're in London, and even in the best case scenario -- water fresh out of the tap -- the nitrates will be "high" as far as purists (and delicate fish like marines and discus) are concerned. Unless you want to use nitrate removal filters, forget about it for now.
Don't add any more fish until your nitrites are zero, and have been for at least 3-4 weeks. Then only add two or three small fish (say, three platies or two swordtails). Monitor the nitrites over the next few weeks, testing once a week at least. As others have pointed out, make sure you do things like use a dechlorinator and don't overfeed.
I hope this helps,
Neale
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I have since learned otherwise thanks to the helpful people on here!
