New atmosphere will create changes in many things. Fish included. That fact that this happened when you upgraded tank size indicates to me that the "bully" is indeed that and trying to establish a hierarchy in the group.
If also have to agree that a 20 is barely enough for 1 and extremely cramped for 3. Even with the most incredible filtration, 25% WC
's are on the light side for those poop factories. For the LFS
to test your water weekly is a great thing but unless they're using liquid test kits, I would throw lots of caution to the wind. They're fairly unreliable in comparison. It's a better chance they're only using the test strips since the overall cost is minimal to them.
My recommendation (although having them for a few years already) would be either a much larger tank or rehoming them sooner than later. It would give them a much better quality of life if the stunting hasn't gravely affected them already.
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