I'd keep in mind that you also should be testing phosphates and nitrates. If your tank is fully cycled, it should never have detectable nitrite or ammonia. Phosphate and nitrate are what indicate excess nutrients in a cycled tank.
Also, for SPS you have to keep all parameters stable. The exact levels aren't as important as stability. That includes salinity (so you have to be consistent with top offs or have an auto top off system), temperature, alkalinity, calcium, magnesium, nitrate, and phosphate.
I doubt your T5 light is too strong, but most SPS corals do best if you light acclimate them to a new light. When you get new SPS frags, you should place them lower in the tank first and then slowly move them up to their final placements locations. Once they're acclimated, you want to provide them with high light and high flow. Generally if your light was too strong, that would have just made the coral bleach (or lose color), not undergo rapid tissue necrosis and die.
Another big considerations would be pests, which can kill SPS corals quickly. You shouldn't FW dip your SPS, but you should be dipping new frags in saltwater with a chemical for cleaning corals, such as Revive or Coral RX. This will help prevent pests from being introduced to your system. You might want to keep an eye out for red bugs or acro eating flat worms.
