Use what your LFS uses – they aren't likely to put up with junk. Buying sight-unseen is always a risk....and that's buying online.
Otherwise just don't buy the cheapest one you can find. IMO if you can see a weight listed, buy the heaviest one you can. The new ones I've touched feel like they're made of styrofoam and feathers.
If you are stuck with shopping Amazon for stuff like this, then firstly I'm sorry you have no LFS. Second, search for "salinity refractometer", set the price range to "$0 to $80", then sort by "Price: High to Low". Buy from the top part of the list. Avoid all the $8 and $15 dollar units IMO. Maybe they are fine, but that's too cheap IMO.
All that said, refractometers are calibrated with a set screw. Folks complaining about their refractometer "losing calibration" make no sense to me. Set screws don't move on their own. But then I also don't know how they are treating their equipment. All I know is that I bought one in the mid-2000's (from Marine Dopot for <$40 I think) and it looks like it's that old, but it has never "lost its calibration". Sounds to me like nonsense.
A swing arm hydrometer is perfectly adequate, BTW, and can be calibrated to be even better than adequate:
If you're leery of other options, this is a bulletproof one. And cost is <$20....and you can freely buy the cheapest one you can find. Get a used one for free somewhere if you can!
But IMO a refractometer is still the best option overall. Faster, more accurate and more precise than a swing-arm, but just as dependable. (Also more expensive.)
They aren't schooling fish, so I'm not surprised. They are also very sensitive, so other fish might be scaring them out of the tank. I'd try keeping fewer and see if that helps. 5 or less IMO. 3 or less might even be better.
Train yourself to hang out in front of the tank for (up to) 10 minutes and they will start to get more used to you eventually. Otherwise they'll always just be reacting.
I wold never ever bother with injections unless MAYBE I was only dealing win a single aiptasia. Even then....too fiddly.
In all other cases (and even with only a single, honestly) I'd use something like Aiptasia X. Simpler, easier, faster and at least as effective if not more so.