Hi Lellynelly,
I've just had a quick skim through your past notes to remind me of the situation and a couple of things strike me, so can I just check a couple of things? This should help us advise you further.
* what exactly did you do for your fishless cycle, did you use an ammonia source (and, if so, what, and how were you dosing it)? [I noticed you were using a paper-based test strip for the first while, and, if I've understood it correctly, it seems that you mentioned moving water from the established tank into the tank undergoing the fishless cycle? There's also mention of a media swap.] Sorry for the questions, but I just want to check that the fishless cycle process, which is essentially preparing the filter to be able to cope with and process the fish waste by building up sufficient beneficial bacteria, is indeed complete and therefore safe for fish - the water readings alone wouldn't be able to tell us that (as actually a fishtank filled with water and left to sit for a few weeks would give a reading as though it's safe for fish but the filter wouldn't have gone through the crucial preparation stage).
* have you any idea how your nitrates might have reached 250 in the 50-litre tank, and how long they may have been like that? Although I'd advise more weekly water changes of 25% generally, and more frequently and larger with the fish you had in your small tank as they grew, I wouldn't have expected the nitrates to be so high with the water regime which you had. There are other reasons for high nitrates though so it would be worth getting to the bottom of that to avoid it happening again. Can I also check what the media swap, which I think you did, involved - did you swap all of it from the 50 litre tank's filter into the 200 litre tank's filter - or how much of it did you move across?
Again, apologies for the questions, but it should help us to help you further and ultimately help with the actual question you've posed. Thanks.