Harlequin Rasbora swim problem

Hi, I have a harlequin raspbora that is swimming strangely. His head is always pointing upwards, tail down and he sort of “hops”. Otherwise the fish seems ok. No discolouration or marks.
He started doing this after my tank heater and filter had been switched off by accident for 7 hours causing a decrease in temperature from 25 to 22/23. I sorted out the temperature by adding some warmer water then allowing it to rise slowly. It didn’t occur to me immediately the effect of not having my canister filter running for 7 hours. The following morning I tested my water and my ammonia was 0.25 (API Test) Nitrite 0 and Nitrate 10.
Normally I have no ammonia on my weekly test. I did an immediate 30% water change and ammonia is now 0 again. Temperature back to normal.
That said this is the third Rasbora that I have had with this problem. I bought them at the end of August last year and noticed one of them swimming like this immediately and returned it to the shop the following day. In November a second harlequin started doing the same and died, well disappeared, a month later. Now this one starts in February.
Does anyone know why this is happening and why there is so much time between each fish being affected. I have 6 harlequins, 6 juvenile rainbow fish, 3 Molly fish, 2 otocinclus, 6 Amano shrimp and 4 Nerite snails in a 245 litres planted tank. The tank has been established for 15 months. Thank you in advance.