I have two 55g tanks at the moment. One is a peacock tank which I got all 12 fish from a very good online Cichlid shop and stocked all those fish all at once. So since they all came from the same place and were stocked at the exact same time there was no need to quarantine anything because it was like they were basically quarantined together. My other 55g tank I stocked over about a 3 week period, with Mbuna from three different, yet clean and trusted local pet shops.
The Mbuna tank which had no quarantine of the fish I have had one beautiful $30 Mbamba bay male die suddenly of bloat. One yellow lab die of unknown reasons (read my other thread) and still have one yellow lab that has a swelled belly yet is doing "okay" still.
I'm VERY careful about water parameters. My PH, is rock steady and other parameters are very steady, I do PWC twice a week, so the strange illness in some of the fish is not from anything to do with my water parameters. My Thoughts are that it is to do with not quarantining the fish. Can I prove 100% that is the reason? No, but it seems to be the reason if you ask me.
I now have a 20g which is now my quarantine tank and every fish I buy from anywhere no matter how trust worthy will now be quarantined for at least 3-5 weeks. IMO I learned a hard yet valuable lesson.