Plants will absorb some ammonia too. When the ammonia reading gets too large, do a 25% partial water change with treated water of the same sort. If you have time to season or let the water sit a few days, open topped that would be even better. Water changes and plants – and the addition of new activated carbon to the filter will all alleviate the dangerously high level of a chemical. That will extend the time it will take to establish a cycle though. Better to extend the time than to mess up the fish. The recommended 25-50% weekly water change is too get the less toxic, but still toxic nitrate out.

And (sigh) guess what happened when we add a couple of new fish to the ones already there? After a quarantine if possible, they will cause another ammonia spike and after that a nitrite spike once again. Every time we change the equation on one end, there will be a change elsewhere and it will take some time for the cycle to repeat itself.

Feeding fish is something of an art. If numbers of ammonia and nitrite are climbing, but are not too high, ease up on the feeding. Or feed what you would in one meal in three.

If you were to nudge your heater up a degree a day until the temperature was 78, biological processes would go a little faster. Don't feed any more for a bit and see if that makes a difference.

I am really pleased to see you monitoring the process with test kits. The pH and hardness measures will not matter a whole lot in cycling (unless the pH gets up near 9) but in your case seem important. 300 ppm hardness is pretty darn hard. You want to be careful not to just top off evaporation with tap water or your water could become fatally hard.

Topping off evaporation or cutting the changing water with demineralized water can be done with reverse osmosis water or the very expensive distilled water or DI water (run by resins designed to take out minerals).

If you get clean rainwater, wait 20 minutes (for the roof and air to wash clean) and collect that. Automotive, and at one time steel, pollution make that usually impossible for me. Because my tap water is often as high as 400 PPM we eventually got an RO unit. I'm cutting my livebearer's water more and more and will soon reach 50% tap/RO. Rain Forest fishes (many African and South American killifish and a few tetras, peacock goby, and the rare Cory) which I wish to breed will get a 75% RO cut from early on.

That accumulation of minerals is not yet something you will have to worry about, but there may be a time when you want to be careful. Guppies are pretty tolerant of hard water. Bettas obviously can be adjusted to hard water, but actually come from pretty soft stuff originally.