It's a good thing I added the heater. I have an unheated 1L freshwater tub in the same room with only java moss, blanketweed and Anubias. Decided to put the thermometer and there found out the water temperature was 13 degrees C/55 degrees F. Thankfully, the opae ula tank is at 19 degrees C/66 degrees F, which while not ideal, seems to be within the range that opae ula are okay in.
I decided to move a single frond of java moss from the tub into the opae ula tank, since I have heard it can withstand brackish water pretty well. It seems to have some blanketweed wrapped around it, which I hope won't infest the opae ula tank (hopefully the salinity kills that off). I will give it a few days and see how it does.