I have a dragon fish...I had 2 that I rescued but the one little one passed away...I assume the ammonia burns he had when I got him were just too bad. :(
Dragon fish can grow to 12"+ (if kept in a large enough tank they can get nearly 2 foot) and will need brackish water to live at their best. They don't have the best vision and they shouldn't be kept with nippy fish...
They will also need a 55gallon+ tank. From what I have read they're best kept alone or in groups..They don't do well in pairs..I don't know much about rope fish, but they are neat. And that is a great deal on a 100 watt heater.
I have smaller round pieces of lava rock in my betta tank..I just make sure to fully cover them with thick moss first.