I moved my 4 danios and 1 WCMM this morning from their tiny 14L (I know!) tank to my newly cycled 90L heavily planted tank. Ammonia and Nitrites 0, Nitrates below 5, temp is the same 22C. All went well and everyone seemed happy right away, exploring together, not hiding away or anything. I checked on them a few times throughout the day (thank you lockdown) still fine but this evening I've come downstairs and my WCMM is acting very strangely swimming up and down and along the glass at one corner. I initially thought he was just playing in the flow but now I'm worried he's stressed. He's 8 years old...have I made a huge mistake?
When you say 'cycled', do you mean you completed a fishless cycle on the tank by adding an ammonia source (a 6-8 week process usually to ensure it's cycled) or adding mature media before transfer?
If so, then it's possible that he's seeing and playing with his own reflection, or learning/exploring the expanded boundaries of his new tank, or possibly getting used to / stressed by the more active danios zooming around now that they have more space. He'd benefit from more company of his own kind, and that would help him settle, but I appreciate that he may be the last of his shoal and you may not want to get any more.