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They are used to the hard water so I'm not worried about the fish but because of the plants I'm mixing ro water 60-40 to get about 140ppm.When Byron asked about fish, he was referring to different species needing either slow moving or fast moving water. The fish you've mentioned need slow moving water so either a filter which delivers a low turnover in litres/hour or a filter with a flow rate which can be turned down.
The species you've mentioned brings another point - they are all soft water fish and you give your location as London which has a reputation for hard to very hard water. How hard is your water, and if it is hard, do you intend using RO water to reduce the hardness?