The powerfilter should be fine, but, as I said, you need good gas exchange at the water surface; a powerhead near the top should do this.
The PH is too low; however, the PH, salinity and temp of the hospital should = that of the tank when you do the transfer. You should then raise the ph gradually to 8.2. Both the wrasse and trigger will need to be treated. If it is velvet, you need to act very quickly, as velvet can kill, literally overnight. Prepare the hospital ASAP and move both fish there. Acclimate the fish to the hospital just as you would when you bring home a new fish from the LFS. Perhaps this has been your problem?
What are fish symptoms besides the dusted appearance? Scratching, erratic or jerky swimming movements? Rapid breathing? Do you have a magnifying glass to look closely at the spots to see if you can make a clearer distinction between a powdery or salty appearance. When I say a salty appearance, as with ick, I mean table salt, not sea salt. Have you seen either ick or velvet before?
We need to make a clear diagnosis because the treatment offered can be very different.