I’m probably the last person to give you advice on how to grow Hoya wallichii with all of the problems that I have had with the plant, but I will give it a try. Don’t fool with the water chemistry. For a long while I was trying to lower my water pH for this plant. I don’t think it liked it. Use water out of the tap having allowed it to set for at least 24 hours to dissipate any chlorine that may be present. Fertilize at 1/2 strength with every watering but water sparingly.
Grow in a net pot so that you can be more sure when it is time to water. Don’t keep wet, or allow to get bone dry; leaves will yellow and fall off very quickly. Needs very warm temperatures above 80°F during the day and above 65°F at night. Needs lots of humidity above 60% at all times, better if it is higher. Light levels should not be too high – at least 18 inches under T-5 fluorescent bulbs.