Hi AmesLea,
Rainwater use and your pH and temp are fine for Cherries.
However, the GH and KH seem a bit too high for just rain water without remineralising it (adding Calcium & Magnesium to raise mineral content).
Either the reading is wrong (could be old test kits) or something else is raising it. Rocks are the usual culprit. Take out that rock, unless you know exactly what rock it is.
GH 11 is on the high side for cherries.
Why are you NOT running the filter with the old media from the axolotl?
You said you let the tank cycle, how was this done? And how long?
I suspect the tank and filters aren't properly cycled.
A cycled tank will show some Nitrates.
Ammonia, Nitrite and Nitrates at 0 is most unusual for a cycled tank. This is something that needs to be checked/confirmed immediately.
You mentioned that you can get your LFS to check your water parameters. Get them to confirm your GH and KH again.
A TDS pen is a valuable tool, so get one asap.
Rainwater is great for shrimps and fish, but it needs minerals added. Otherwise your livestock will not do well. So products like Salty Shrimp GH+ is a must and is easy for newcomers to use - just follow the instructions on the packaging.
Inspect your rainwater collection system.
Are your tanks metal? Is the roof metal? Any rusting?
Is the gutter clogged with debris?