wow! you have a lot going on in that tank. First thing you need to do it go out and buy a test kit (liquid is best, but strips will do in a pinch). How long have you had the tank running? Was it cycled? Did you purchase the baby convicts? I don't know much about cichlids and the more aggressive fish, but I'm pretty sure that the 2 JD's and the flowerhorn will not cohabitate well in that situation for long. JD's have a tendency to be very aggressive, and will bully each other and the other fish. I think you will need to re-home some of the fish, or expand to more then one (possibly 3) tanks... I hope someone with more experience jumps in here.
From experience at the nature center, a few of the convict fry will survive and thrive. they are tough little fish, but they can get very territorial in the tank, especially once they start breeding. Definitely get a test kit of some sort (the cost of the different kinds of strips you would have to buy often totals up to more then the cost of the liquid test kit, and the liquid test kit is more accurate). it doesn't matter what your stocking is if your water quality sucks, so you need to be able to check that. Then make sure you have plenty of hiding spots and visual barriers for all of the fish. make sure you have caves and such (can be elaborate store-bought stuff, or as simple as a NEW terracotta pot split in half and placed on the gravel so it makes a cave). Before you go setting up a brackish tank, do your research and get the things you need to keep this tank running healthy first.
Good luck.