By "turns out my alkalinity is very hard" Do you mean that you carbonate hardness (KH test) is very high in your tap water? Which would be good for you (right now, and perhaps in the future).
Were those 5 different tests all in one source water, or did you try say, testing your tap water as well, and got a 'no reading' too? At any rate, those little test strips generally really suck, and are all but useless. They don't seem to be real accurate (at least the ones I had aren't...Hatch Co. Aquacheck pH test strips from Aquatic Ecosystems...I never got a 'no reading' though).
Sheesh...and I thought my test strips were crappy...
hmm...it just says "alkalinity"? Maybe that's synonymous with carbonate alkalinity or carbonate hardness (KH) over there in Florida?...it doesn't say any more anywhere, like on the little piece of paper inside the box? At any rate, I'm thinking that they mean your water is (KH) hard. So, by topping up with that source water, you are buffering up the system a bit...which is good right now. (Which was the conclusion you yourself came to about your top up water :)
Update: Got a proper pH test. It was low, about 5.5. I've been adding the potassium bicarbonate that arrived and I've also put a pile of marble chips in with the fish. I expect it to take up to 5 days to get the pH to about 6.8. Water is clear. Food is eaten. Nitrates are about 60. Nitrites are 0. Ammonia is between .25 and .5. I'll keep a close watch on it.
Thanks again for all the help.
Well it's good that your back on track. And just in time too! It might be good to slow down or stop feeding entirely for a day or two.