When I first started out, I went by a rule of 4g/inch of fish.
When I had my tank stocked, I had probably closer to 2g/inch, but this doesn't apply to all fishes.
No problems maintenance wise, and no issues with fighting. But I had mostly wrasses who tend to wander the whole tank.
When I had a clown and a bangaii, they became aggressive with age and started pushing everyone around. In this case I'm sure it's due to not enough territory so these kinds of fish need more space.
With freshwater fish, the more crowded a tank is, the less aggressive it can be. But with SW fish, at least in my opinion, overcrowding causes stress.
There's a big difference between having a shoaling group of fish who are bunching up for protection, and a group of schooling fish who are happy to just hang out together.
Just like us, they need elbow room, and space to get away from each other.
30 fish in a 50, regardless of sump size, is way too many.
It's not a matter of filtration/water volume capacity, it's all about floor space.
A big sump is awesome for extra capacity (and is what I use), but has no bearing on the fishes 'mental health', at least IMHO
Put yourself in a room 10 x 10 with 1 other person for hours on end. By the end of the work day you're tired of that person, but you get to go home.
Then try it with 20 people for months with no breaks ... would you go crazy ?
I wouldn't put more than 10 fish of a max size of 2-1/2" in a 50, and that is still too many for most that need swimming space.
What you need to consider, instead of volume or fish size, is how much space does each individual fish need. Some need to swim, and some like to stay in one area.
So, in your tank, you've 36" x 18" of space before 'scaping.
Subtract all your LR that takes up the footprint of your tank, and an internal overflow if you have it.
Now what do you have left ? 24" x 6" maybe ?? ... hope you see where I'm going with this.
In this case, you only have 1 square foot of space for your poor fishies to swim in ... !!!! (24*.5/12)
BTW, with my thinking above, the area above the sand to the surface doesn't come into play. Just the open space is considered.
Look at your overall footprint after 'scaping and come up with a square foot measurement.
Then think about what kinds of fish would be happy with that limited amount of space to meander through.
Some will be just fine with never roaming, such as firefish (in my experience), and others need room to rocket around your tank. Think about fast swimmers instead of 'dither' fish in this case. I think about small tangs, foxes etc in this category.
When I finally settled on my fish, I had more wrasses than any others and they fought with each other occasionally.
Mostly the squabbles were between the ones that pick at LR all day, such as a Coris or a Melanurus. These kinds of wrasses can be very territorial as they 'graze' all the time for small morsels/critters and will fight with each other for 'space'. In my 75g I had 1 of each and they did not get along at all !!
Other things to consider are fish with the same habits/diet.
I had a yellow eye kole tang who eats film algae. He constantly chased my starry blenny who has the same diet. Once the kole tang was removed from the tank, the blenny became a prized critter with lots of character and is still around now.
Hope you get it sorted out and some of what I've typed is useful
