Freshwater fish are constantly urinating, so if you are over stocked, the only thing that will keep your tank stable, and from becoming a toilet ( having a pH crashes) are large frequent water changes.
And because you really didn't have much biological filtration going on, a fish die off was bound to eventually happen.
I didn't realize you were so overstocked.
An overhead planted sump was a great suggestion, but even with that, if plan to be so over stocked, you need massive amounts of filtration, and should never slack off on water changes.
Here is an overhead planted sump I use, it is not complicated, and took very little time to set up.

These plants (papyrus) are about 4ft tall, to offset overcrowding you need heavy, thick plant growth.