Reverse osmosis water is water that has had everything that was in the original water removed. it is pure water.
When people with hard water want to keep soft water fish, the hardness has to be reduced. The way to do this is by using RO. Either tap water and RO is mixed before it is added to the tank to lower the hardness, or all RO is used with some minerals added back from a pack.
For example, your 300 ppm water, if mixed half and half with RO would give a GH of 150 ppm. Or one third tap and two thirds RO would give a GH of 100 ppm.
When people with hard water want to keep soft water fish, the hardness has to be reduced. The way to do this is by using RO. Either tap water and RO is mixed before it is added to the tank to lower the hardness, or all RO is used with some minerals added back from a pack.
For example, your 300 ppm water, if mixed half and half with RO would give a GH of 150 ppm. Or one third tap and two thirds RO would give a GH of 100 ppm.