Tango,
How often do you feed your lionfish ? Since these are predators, they do tend to eat more than a large clown and they will produce more wastes that a normal large fish. So, your tank may have more algae problems due to the excess wastes.
Lionfish are treated as reef safe and it should be okay. I would try to put some calerpa or macroalgae in the tank and see how that works out. Some feather or halimeda algae might make the tank rather interesting looking and help control the nutrient levels. I am sure local reefers around your area will have some of these algae strains. You can add some some mangroves too
Conches are only really good for diatoms and cyano. They don't touch the green hairy stuff.
- Victor.