Most fish cope fine with London water, I agree with CFC and Bloo that German/Gold Rams and Apistos don’t do well. Bolivian rams seem to do okay though. I haven’t had trouble with most of the tetras, I kept cardinals and rummynose in London water for a year and a half before they got RO water. Many of the rainbow fish love harder water and of course the African lake cichlids need hard water.
Some plants like the one I mentioned will grow in low light, Egeria Densa will grow in not very low light and hard water, its sold for goldfish tanks in most petshops, its often called pondweed. As CFC say it would need higher light to really eat up the nitrates. But its tough and cheap so you could give it a try.
Have any of the other fish had problems besides the cichlids, if so its probably not the nitrates or the hardness of the water.
Some plants like the one I mentioned will grow in low light, Egeria Densa will grow in not very low light and hard water, its sold for goldfish tanks in most petshops, its often called pondweed. As CFC say it would need higher light to really eat up the nitrates. But its tough and cheap so you could give it a try.
Have any of the other fish had problems besides the cichlids, if so its probably not the nitrates or the hardness of the water.