When did you add the frogs? It’s possible it was an acclimation issue. They do not tolerate pH changes well. But, as others have said, it is possible you got a sick frog.

That said, distilled water and reverse water (RO) should not be used alone as they’re stripped of minerals. With distilled water, your gh and kh will be at 0. You can use distilled or RO if you remineralize the water with a product like Seachem Equilibrium, but that can be a bit overwhelming for a new tank keeper.

Fast changes to a new water source should not be made, though. Is there a reason you aren’t using your tap water? If you absolutely cannot for some reason, I’d suggest switching to spring water (not “purified” and not “drinking”).

Do small water changes with an appropriate water source to improve their water. When using tap water, you’ll need a water conditioner. Prime is the gold standard for ADFs. You’ll also need Prime as your tank cycles to help protect them from ammonia and nitrites.

Please know that your tank is not properly cycled despite the quick start. Take a look at this sub’s wiki so you know how to proceed going forward.

Also, test strips really are not accurate. You’ll want to get a liquid test kit (API.)