Douahe, your tank is inappropriately stocked. What are these "platinum gouramis"? Divide your stocklist in this manner but bear in mind you either ditch some of your fish or move to other tanks.
Option 1
2 bala sharks,
2 rainbow sharks
2 red tail sharks
Option 2
1 gold veil angelfish
1 black veil angelfish
3 swordtails
13 saiamese algae eaters
2 cory cats
2 pearl gouramis
1 red dwarf gourami-
Ditch the angelfish and pearl gouramis if you want this fish.
Option 3-125g
1 black ghost knifefish
2 clown loaches-
Add more clown loaches. This is strictly recommended.
Option 4-A 30g is minimum.
2 platinum gouramis
Black angelfish-It is hardly surprising that they are a trouble to keep alive.

These are one of the few strains of
Pterophyllum scalare that is very sensitive to water conditions.
Dwarf gouramis-I've discussed many times before that dwarf gouramis, if your source is from Asia, are fed with foods laced with hormones to change their sexes. Males are in high demand so you must expect that searching for females is very slim. Fish farms in Asia will hardly care for the quality of the fish. It's all about demand. I live in Asia myself and have received complaints from my local petstores about these dwarf gouramis. The effects of the hormones unfortunately allow the gouramis to become very vulnerable to several illnesses and shorten their lifespan. Again, this is hardly surprising yet not many people are aware of this.
Tank Maintenance comments:
I see you change your water every 2-3 weeks. It is no wonder you have a hard time keping black angelfish. Your water changes are very inadequate. If your schedule permits, do a water change by 50% twice a week. if not, weekly but the more frequent the water changes, the better.
Water parameters:
Post the exact results of ammonia, nitrites, nitrates and pH. Comments alone do not suffice.