Hopefully the fish has just damaged itself as you say, mine do that sometimes, although the changes in the water will undoubtedly have caused them stress and the water they prefer was the water you started off with? There are sections on this forum dedicated to fish, I used it when I decided to try a Betta and got great advice from JayC and this Betta is doing way better than any I have tried before because of all the different things I did/do this time!
Great to hear that the shrimps are doing so well. I think it is natural for the berried shrimp to hide away when they are near to birth as it protects the newly hatched shrimps when they are born, and the babies don't move far when they newly hatched either. There would be little to gain at this stage in getting more shrimps before getting a bigger/new tank, it may even be detrimental?
I admit to being lazy sometimes and not topping up the tank between weekly maintenance/water changes (disgraceful really as I keep a 1litre bottle of RO water right next to the tank anyway), in which case I end up taking out 2litres and putting back 3litres at the next change, but need to make an allowance for the fact the TDS will change. When I set up the new Taiwan bee tank I measured the water I put into the tank at the start (assuming the measuring jug is anywhere near accurate) so that is how I know there is 28litres in the 35litre tank - it is a good idea to do this WITH A NEW TANK (but with everything in the tank, heater, filter substrate etc) as it is so much easier to do at that point anyway. I did the same with the Betta tank and that has 20litres in a 25litre tank (the one you were thinking of getting at some point). Without the exact figure to put in the spreadsheet then it won't be 'accurate (can still be used to give you as near an idea as needed though)', especially on a small tank! It works to the digit with mine. Even if you empty the tank and refill it probably won't be that accurate as the soil will have absorbed some water, so best do it first fill of new tank.
Your water parameters are looking good and pretty much there now so all else being well you should be able to go back to once a week maintenance/water change. I don't know much about Nitrite/Nitrate as I have never had any readings, apart from the old fish/cherry shrimp tank which has always been 50 nitrate for years, probably because there are too many fish but there has never been a problem in that tank? PURELY AS A GUESS I would think the duckweed may have caused the problem/imbalance, but that is ONLY a guess based on I used it once and swore never again, I use water lettuce now, though that isn't perfect either as the routes grow so quickly that I have to trim those every week, but still a lot easier than duckweed.
Thanks JayC, I think it is called a Geode, that was what it was sold as when I bought it in US many years ago though it could be something else? I think it is some sort of meteorite?
Hopefully, as you say, now it is removed the PH may start to drop and I have put one in some prepared water to test in a week. I will also just see what happens with prepared water left out for the same period just to see what happens with that.