Water softeners work by replacing calcium and magnesium with sodium bicarbonate. Sodium bicarbonate will push the PH up. More plants will push the PH up even more because plants consume the carbonate resulting in Soodium hydroxide which pushes the ph up even more. Additionally most steams and lakes always have some sodium and som potassium. Water with just or sodium or mostly sodium is bad for fish health.
100 gallon per day RO system
This is the fastest and fastest RO system I know of. For your 75gallon you might want to consider the 50 gallon per day version. Us a commercial GH booster or make your own. with
calcium gluconate or
magnesiumgluconate. Natural was also always has calcium and magnesium which fish and plants need. (two talbspoon of calcium gluconate and 1 of magnesium gluconate and and mix in a container add only enough to your tank to reach one degree. Unless you have fish that need harder water. Ca Mg gulconate may resolve slowly but most should be dissolved within 30 minutes to an hour or less. Many commercial GH boosters have a too much CL or sulfur which may make the water a little acidic. Gluocnates don't do that.
Also put a sea shell or crushed coral in the the filter. This will keep your water very close to 7 if too much Cl or sulfur gets into the tank. Amazon sells calcium gluconate and magnesium gluconate. You will need to use a fertilizer if you have plants. in the tank. So a water change once a week.
