A good RO
Unit will strip your water of just about everything, including buffering agents that help to keep yor PH from crashing. When your PH Crashes, fish can die....and this can happen very quickly. Make sure you have a good TDS
Meter to measure the dissolved solids in your water. I used to use Kent RO
Right Powder to put some necessary minerals and such back into my RO
Water, but now I just mix my RO
water (85%)...with my hard well water (15%)
The fact that your tank has been established for a while will not help you buffer the water, unless your substrate is one that is meant to add hardness like crushed coral or some of the other gravels made specifically for African Cichlid setups. A RO
Unit is a great thing and I love mine....it helps me keep the soft water fish that I love without hauling water in on a weekly basis. I keep Angels, Discus, Plecos, Cardinals, Rummynose, and others. Plus I can still use water right out of my well to keep my Malawi and Tanganyika Cichlids as well. Good luck!