Yes, I do use RO/DI (I assume you're talking about reverse osmosis/de-ion right?) I have about 500 gallons and I change 60 gallons a week on average. You could try to use live ghost shrimp for the time being to fatten her up. I'm actually a little concerned about my male. He eats, but he's not as excited about food as I'd like him to be. I'm going to start soaking his food in garlic tomorrow to see if that encourages him to eat. You could try that as well, because I'm sure force feeding is a pain in the rear.