If you still have the platy it won't last very long in pure RO. The 75/25 suggestion is actually not a bad one although with the platy you may want to think about 2:1, or even 3:2.
RO comes out of the tap at the same temp as tap water - which is way too cold in winter. It would also take too long. My 100GPD unit takes around 6 hours to produce 100l - which is my weekly change in a similar size tank. This weekend I had had to heat my water for the water changes - first time in a while. I store mine in 25l jerry cans for the big tank and 10l jerry cans for the smaller ones. Currently filling the last one (25l) after todays 100l change. Usually I refill immediately after the water change so I am ready for next week. No need to dechlorinate because chlorine kills RO membranes, so whatever system you have will have a dechloriation stage before the RO membrane.
RO comes out of the tap at the same temp as tap water - which is way too cold in winter. It would also take too long. My 100GPD unit takes around 6 hours to produce 100l - which is my weekly change in a similar size tank. This weekend I had had to heat my water for the water changes - first time in a while. I store mine in 25l jerry cans for the big tank and 10l jerry cans for the smaller ones. Currently filling the last one (25l) after todays 100l change. Usually I refill immediately after the water change so I am ready for next week. No need to dechlorinate because chlorine kills RO membranes, so whatever system you have will have a dechloriation stage before the RO membrane.