A 300 might sustain one for juvenile to sub-adult, but certainly not for life.
Not to be rude, or to start an arguement, but you've had yours for what?...a few months (6 months, at most), unleashed?...and it's only 30" or so? That's hardly an adult sized Tesselata. That's barely more than a baby.
You may say that they'll only reach 5' 10", but myself and countless others have seen Tess. that have been close to 10' in length. I've personally seen 20+ different specimens (in captivity and in the wild) that have been larger than 8'. You also say that it's very unlikely that they'll reach full adult length, in captivity...but, just like any other fish, if you provide enough space, feed right, etc. then it WILL reach adult length (alot bigger than 5' 10").
If you research and dont think you can properly house a Green Moray, then you cant properly house a Tesselata Moray.