I commend your bravery in trying this, but
I realy think you are adding fish way to fast with this small of a tank.
I had a 5.5g nano running for 4.5 years, I am pretty familiar with them as reef set ups. the smaller the tank the harder to maintain. Bigger salenity swings,temperature swings, and amonia , nitrites, and nirtates build up realy fast.
each time you add a fish to that small of a body of water your tank cycles again. Even with a tiny clown fish,or a micro small chromis.
Especialy given the fact that your running with no skimmer, no rock and a bare bottom.
The next one to die off on you will be the baby lion more than likely.
From stress of a small tank, and no bacteria built up yet to nuetralize the amonia out put from all those fish. Lion fish have huge apetites, are very messy eaters,and very messy deficaters. But when they are that young they are very hard to feed (as you are finding out) the stress of so many fish in such a confined space will consantly freak him out. I fhe is constantyl stressed he wont eat much at all.
even by himself he would crash the tank IMHO.
You are right that bubble filter will maintain the tank up to a certain bioload, but you have already surpassed that by 3 fish.
If it were me I would Stuff it with live rockbut no substrate.
The rock would be your filtration along with FREQUENT water changes.
I would add a mini jet 404 if I were you as well.
Your going to need all the water movement youcan get tokeep the fecies,and debries a float long enogh tomake it through the bubble filter.