PH of 7.5 and Kh 4/5

Your kH is fine. No need to mess with it.
The best way to lower pH is with CO2. You can inject with no problem with that kH.
If you don't want to use CO2 you will probably be fine for growing low light plants in that environment just the way it is.
I would not use chemicals to lower pH, especially when it is not necessary.

Len

Angelfish will adapt fine to a ph of 7.5... Your tank is really too big to use the DIY CO2. You would need to go with the pressurized.

I agree with Tempest. Your angels will adapt to your water conditions. This will be made easier if the water is kept clean. By that I mean regular water changes. I only add fish just after a water change. This has, in my experience, made the acclimation process much easier on the fish I've kept over the years. I also take a lot of time adding them to the tank. I brought home some Rams and Cardinals yesterday at about 12:30 and they didn't go into the tank until close to 6:00PM. I add tank water to their bags in small increments to lessen any pH shock. Works for me.
I also agree with Tempest on the CO2. DIY is really only affective on tanks of up to 30 - 40 gallons.....Max!! Unless you want to set up 5 or 6 bottles at a time and daisy chain them.
For example, when I was using DIY on my 55s I was using 4-2liter
bottles on EACH one to get the affect I wanted. Mixing all that sugar and yeast gets old really quickly. Now, I only use it on my 10gal. isolation tank, which is also planted. Works great on a tank that small, but not the larger ones.
Here's an article on DIY CO2 if you're still determined to try it:

http://www.animalnetwork.com/fish2/aqfm/1997/oct/aquatic/default.asp

Yes, I am a dj.

Len

You do not need to change your pH for angels:

pH 7.4, gh 6, kh 5


You can lower pH using peat. However, you will find that it is hard to maintain a stable pH at a given level, especially at water changes.

I do not modify my water params, here is what I have in 7.4 pH:

Angels
Discus
Clown loach
sidthimunki loach
montezuma swords
harlequin rasboras
cardinal tetras
assorted corys- panda, similis, sterbai
assorted plecos- dwarf bristles, zebras, L-134, L-333, queen arab
SAEs
oto affinis and zebra
denisonii barbs
threadfin rainbows
furcatus rainbows
z danio
white cloud

Fish I have had but no longer do
Rosey barb
Cherry barb
Dwarf Flame gourami
betta
red tailed shark
yoyo loach
common swords
false SAE

There are many more you can do as well.
paleatus cory
gertrudae rainbows