I wouldn't try sterbai any more in harder water as they seem to be getting weaker strains imported.
Corys don't do that well in hard water anyway, considering their lifespan should be about 20 years...

If you really want to keep the fish you keep listing then you'll need to make sure you have
a) the right sized tank
b) the right water, you'll have to play around with RO water mixed with your tap water to get the softness required
..or go for hard water fish like the smaller rainbows....

Sorry to hear.

Ammonia and nitrite must be zero before it's finished. If you have any of either it's toxic to fish, shrimp and snails....

Sorry to hear about the skunks.
Glad to hear about the rest though.

Weekly, unless my back's screwed....

As well as being too soft it's also way too warm, I keep my guppies at around 20C and 22C is the tops for them.

Either will do.

Give her (she looks like a she, no bristles) some fresh veg like courgette and also a little crushed garlic (less than 1/4 clove), that should build up the immune system, just in case.

Welcome to the forum. :)
If the water is (only) 7dGH then it's a bit softer than mine (7.8), and tetras will do ok as a rule. Not sure about diamonds though as they will usually need soft water below 6dGH - I wouldn't keep them. They do all need different temps though...
I'm only wary of that figure ...