The white stuff won't be causing the problem. They could be hydra, worms or algae but they don't cause problems to adult fish.
What happened to the other fish?
it might be worth contacting the shop you got them from. They may have had a problem with their batch of Harlequins and if not (or they don't admit it) they might test your water for you and, if all shows ok to them, they may replace the fish.
Most of the shops around my way do this so you might be in luck.
I'm pretty sure they are being poisoned.
When you do water changes, are you using fish only buckets? Basically a bucket that has never been used for anything else?
How do you remove the tank water?
Have you got carbon in the filter?
carbon is a small black granulated substance, usually in a mesh bag
Yeah have done, waiting on a response, when I called the other day they said they didn't do returns. Its a Maidenhead store FWIW.
rinse the carbon first to remove any fine black dust, then put it in a bag and put in the filter. If you don't have a fine mesh bag then put it between a couple of layers of sponge.
Wash the fish buckets out with warm/ hot soapy water a few times. Oily residues & chemicals won't always come out with cold water.
Do you have anything on your hands, moisturising cream, oil, grease, medication/ ointment?
What sort of soap do you use to wash your hands?
Do you use a disinfectant to clean the tap handles?
Perhaps try washing the tap and tap handle with soapy water a few times before working on the tank or getting water for a water change.
Rinse your hands and arms down with water before working in the tank.
Are you using tap water for the water changes?
If using well water or rain water it could be contaminated.
I would do another water change tomorrow. Make sure you double dose with conditioner and dechlorinate the water before adding it to the tank.
See if you can increase aeration too, more oxygen can help.
If you have a clean sponge that has no soap or additives, then wipe the inside of the glass to remove the white stuff. It shouldn't be causing the problem but if it comes back tomorrow, then there might be something in the tank that is being released into the water.
Interesting. My local Maidenhead Aquatics are really good with things like this. If you take in a water sample they'll test it for you and if it shows up as ok but you lost fish they normally offer some fish to you.
If you have a test kit, check the tank water for ammonia, nitrite and nitrate, and check the tap water for the same thing. If you can't test for those items take a sample of tank water and tap water to the shop and get them to test it for ammonia, nitrite and nitrate. Label the tank water and tap water so you know which container is which.
Write the results down when they test the water. Post the results here. We want actual numbers not "it's ok".
If the shop offers to replace the fish, do not take any home yet. Get them to give you a store credit until this is resolved otherwise you could add new fish and lose them too.
Do you use a hand sanitiser at work? Some of these leave a residue on your skin and can be hard to wash off. I use an alcohol based hand cleanser and have to wash my hands 3 times with hot soapy water before I can no longer taste the residue on my skin.