Here is my experience: I have a 70 gallon tank with a LOT of LR ( I have no idea how much but more than 75 pounds). I used to have millions of pods in my tank. HUGE ones. All different shapes and sizes. I'm talking bugs that make you stop and call the wife to say, "Hey come and look at this!" I'd see them both lights out and even during the day. I have had a spotted mandarin in the tank for about 16 months now. It is doing fine. BUT, (and this is a huge but), I no longer see ANY pods in my tank at all, day or night. None. Zero. Zip. This tells me that they eat a LOT of pods, basically she is eating the pods in my tank as fast as they reproduce as I see no adult pods anymore. (Babies either for that matter.) I assume the ones reproducing are very good at hiding. What she is eating, I have no clue, but, a 39 gallon is seriously unlikely to be able to support a mandarin IMO.