Obese axies - how to put them on a diet?

My guys and girls are in serious need of a diet. They've been living with my dad for a few years because I've not had the space for them and after about 6 months, I visited them for the first time and they were COLOSSAL I asked dad what he'd been feeding them. Turns out they'd been eating nothing but ox liver. So I chastised him, bought some proper pellets and thought that'd be the end of it.
Well, they're coming home tomorrow so I asked dad if I'd need to buy them any more food, and it turns out he's STILL been feeding them ox liver! And don't even get me started on me telling him the difference between the sexes and them still living together...
So now I have 5 obese axolotls who are desperately in need of a treadmill or something. But how do you diet an axolotl? I have pellets for them but how much shall I cut their intake to?

If they are adults, feed them every 3 days. Ox liver is a really poor food choice for your axolotl.

I know I'm really worried about heart disease. And it's not like they're getting any younger. The oldest 3 are coming up to about 9 now.

I would be much more worried about high levels of Vitamin A from the liver than I would heart disease.

Tell your dad that it's really not a healthy diet, and they need to be on something more balanced.