If you are serious about SPS before you go a change anything, you absolutely need to have an extremely accurate method of measuring Alkalinity and Phosphates. For phosphates nothing beats the Hanna HI-736 which reads in parts per BILLION. The API tests are absolute garbage and completely worthless. You do NOT have 0.00 phosphates - there is no way. API will test 0 phosphates with levels up above 0.25ppm which is more than enough to kill off acros. Even the Salifert Phosphate kit will test 0.00ppm up to around 0.10ppm or so. Hanna is really the only way to go for Phosphate.
For Alkalinity, you need either a Hanna Checker or a Titration style test kit (Salifert, Red Sea Pro, Lamotte, etc.).
In your case, there are absolutely no changes you can make to your tank besides flow that will help you keep your acros alive unless you know exactly what is going on in your tank. While you are at it, invest in a quality Magnesium test as well since that is important too (though that tends to fall in line if Alk and Ca are correct).
If your acros are not colorful it is certainly not too little light and NOT too little nutrients - 3x MH is plenty. Flow may be lacking, but that isn't going to change the coloring all that much, that'll more determine growth patterns and branch thickness. You absolutely need to know accurately and precisely what your levels are. API test kits are 110% garbage for this.
Before buying anything else, get quality test kits since without them you are just dumping money in the dark. Additional flow can never hurt (like am MP or Tunze), but that isn't going to color anything up.