replace high KH & high pH water w RO/DI water

Question
QUESTION: Hi there, I have a 55 gallon planted tank that I want to decrease the KH & pH because I desire to keep angelfish. Currently my parameters are:
KH:11
pH:8.2 - 8.4.
Nitrates:0
I live on a well system and up to now have used that water for water changes (thus the above parameters). The tank has been up and running for over a year now and have 2 dwarf gouramis (doing ok), 2 SAE (doing great), 3 otocinclis (ok), 2 glass catfish (their mates have slowly died off but the remainder are doing well)and a male & female betta (doing well). When I say doing ok, I mean that they are feeding & eating but just look a bit off-color and not all that energetic. I also have a 120 gallon reef tank so I have a RO/DI unit and am fairly well-versed in keeping parameters stable. My question is how slow (or fast) should I replace the current water w/the RO water...I'm sure slow overall but what would be acceptable daily, weekly, ect? I've attempted to acclimate angels to my current conditions and no matter how slow I do it, I have not had any luck. Oh, one more thing that may be also affecting my calcium level...when I originally set up the tank, I had used leftover crushed calcium from a ciclid tank as the base (about 3/4"). I have over 2" of substrate (small black rock)over that...will this continue to leach into the system negating whatever changes I attempt to make now?

ANSWER: Hi Wendy,
Angelfish are quite hardy, I believe a straight 50% water change from the current 8.2-8.4 pH to an acidic environment shouldn't harm them.  I dont think all that weening is necessary.  When you said you had no luck, were you talking about experiencing adverse effects when weening them into an alkaline environment?  If you use well water, I'm surprised that the pH is so high.  Usually all wells have a very low pH, around 6.0.  Also when you said you used crushed calcium for the bottom on the substrate, did you mean crushed coral?  It is very easy to raise the pH of the tank but hard to lower it.  I believe if it is crushed coral which is used in a lot of cichlid tanks then that might explain why the pH is so high.  I would remove the coral if you plan to keep angels.  It will always leak into the water column raising the pH.  Im not sure how long it will do that, but I believe it will take a long while.
-Matt-

---------- FOLLOW-UP ----------

QUESTION: Matt, thanks for your answer! I had problems when I first introduced the angels to the tank...the koi angels died within 2 days and the black marbles were doing great for a week then all died within 24 hours. I have no surviving angels just the fish I mentioned above. Yes, i did mean crushed coral. Even my well water straight from the tap has an alkaline reading...about 8.2 - 8.4 (I used 2 seperate test kits). I will follow your advice about the 50% water change with the RO/DI water and then monitor for a bit. It would be impossible to take out the crused coral as all of my many plants are well rooted, plus there is so much substrate on top that it would be a disaster!! If I don't have luck then I guess I will look for fish that relish my conditions. thanks agian  wendy

Answer
No problem Wendy,
That's true that it would be quite hard to remove the crushed coral if it is under the substrate, especially with all the plants.  Angel fish should be fine in pHs 7.0-7.6 but breeding is more successful in a lower pH such as 6.0-7.0.  If you plan to breed Angelfish, be sure to use a flat white tile leaned slightly agianst the tank.  I know a hobbyist that breeds angels and she uses 1/3rd or 1/2 of a tile leaned agianst the tank for the angels to lay eggs on.  The tile is supposed to imitate the Amazon Sword found in their habitat.  It is also cleaner, bright, and helps the Angelfish care for their eggs easier.  
-Matt-