Time to wade in on this one! Neither 'balance' nor 'ecosystems' exist in this context. Nature is constantly changing; big changes little changes, cataclysmic changes. Species went extinct long before we came along and will continue to do so once we are gone. Defining any part of the ocean as an ecosystem would require the ability to define the limits and since all oceans connect, and rivers from land feed the sea, the idea of a closed system is misleading. That, however, is rather beside the point.
I work in Grand Cayman and last year we had no lionfish. This year we have caught ten off one shore site alone with reported sightings of another eight. About 150 have been caught island-wide so far as I am aware. Some are now very close to a six inch body length which means sexual maturity. I was a bit cynical about the whole culling programme, it's not the first time an invasive species has been dropped in the wrong place, but duly went along to the talk. The numbers are pretty bad news for anyone who makes their living off the water.
It is not a certainty that grouper do eat lionfish, a couple of grouper have been found with lionfish in their stomachs but not many and one in captivity at a lionfish but it may have been conditioned to eat anything dropped in the tank. It takes 4-7 years for a grouper to reach 400-450mm in length at which time it can reproduce. A 1.2m grouper will be about 30 years old. It looks as though a lionfish in the Caribbean will be sexually mature in six months and for some reason (warmth of water, amount of food) they get to be 50% bigger than in their natural environment. The largest caught so far was 470mm long. Farming and releasing Grouper will be a very slow process. Incedentally, in their natural environment lionfish do not have any major predators either, the limiting factor on their population is not well understood. Since the Red Sea only got them when the Suez opened up, the answer may lie there.
They can eat fish about half their body length, maybe a little more. They eat anything! The first reports have come from the Bahamas that some have been found with baby lobsters and shrimp in their stomachs; they have cleaned the reef of other fish. We were also shown a photograph of a dissected lionfish that had 21 juvenile grunts in its stomach. They digest very quickly so that would be one day's eating. They can also fold their fins back and get into some surprisingly small holes, in fact they prefer to hide in holes especially if they've been harassed.
A female is able to lay some 30 000 eggs a month, every month in water this warm. It is possible that in the Pacific something eats the eggs but that is not the case here.
One thing is for sure, they are here to stay. The best we can do is catch as many as possible to slow the spread and enable the other species to learn to live with them. If they carry on at their present rate, they will wipe out a number of species beyond their capacity for recovery. As some of those species disappear the reefs will go too. Since more than one industry relies on those reefs and associated life we should make some effort to limit the spread. From what we know, they are all genetically very similar so I'm all for introducing a mutation to limit their reproductive ability. It's working for mosquitoes and they are notoriously difficult to spear.