Unfortunately I found my last remaining ADF dead this afternoon. I'd been doing daily water changes since receiving my initial advice on this thread while I wait for my new tank to arrive but sadly it wasn't enough, maybe it had come too late. Just have the 3 cories left now, tank should arrive in the next couple of weeks so am just going to keep up the daily water changes until I can get them in the bigger tank. I won't rush to get any more until I'm happy things have stabilised.