Bad To Worse... Please Help!

Unfortunately the only way to test for bugs is at a vet if you could find one willing to do it as most of them know little about fish.

I would suggest the nitrite is to blame, though can I ask which tester you are using? Most of the ones I'm aware of only go up to 5ppm (mg/l are the same thing).

If it is a genuine reading, nitrite is extremely high, high enough to kill fish. It blocks the oxygen receptors on red blood cells in the same way carbon monoxide does to us and the fish suffocates. The nitrite does not come off the blood cell, which is why the effect is long term. The affected blood cells are eventually broken down and replaced in the same way that all blood cells are.

The best thing to save your remaining fish is a huge water change, or several big ones in a row. Test half an hour after each water change (to allow the new water to mix in) and keep on doing changes until nitrite reads below 0.25. Then test every day and do more water changes when necessary to keep it below that level.

But why is your nitrite so high? It could be the colony of nitrite eaters was disturbed during the move.

It is possible that the new fish did introduce something and the combined effect of that and the high nitrite has killed the old fish. But the nitrite alone would have a bad effect on both new and old fish.

Oooh, a shop that says strips are rubbish instead of trying to sell them to you.

Zero nitrate is unusual unless you've just done a big water change with water that doesn't contain any - and that's unusual in the UK. Some places have 40ppm in the tap water. With nitrate being the end point in the nitrogen cycle, a tank usually has a reading somewhere above zero for that. But nitrate liquid testers are not terribly accurate. If the shop man didn't shake one bottle hard enough, it would have given a false reading.

How do introduce new fish to the tank? The better way is to float the bag in the tank and add small amounts of tank water every 15-ish minutes for a couple of hours to gradually change the bag water to the same as your tank.

Do you have a combi boiler or a hot water cylinder fed by a header tank in the attic? With a combi boiler, it is OK to use hot tap water. With the hot water cylinder, there could be anything in the header tank in the attic, so you should boil a kettle.

Adding a small volume of RO at each water change will lower your hardness, but if it's only a tiny % of the total water change not by much. Be careful about changing the water conditions with RO too much at any one water change.

Melafix is more akin to dettol for us - fine to stop an infection getting in a wound but not much use once an infection has taken hold. Be warned, it smells of tea tree oil!