Odds are, they have fin-rot do to injuries from aggression. Every fish in your tank is totally incompatible with a betta. Also, to the best of my knowledge, tetras and angles do not mix, for the same reasons as bettas. The smaller faster fish will nip their fins, which leads to infections.
Your best bet would be rehousing the betta in a smaller tank ( at
least 2 gals ) asap. As I don't know much about what can and can't go with gourami I cannot help you there. Hopefully some-one else will be along that may know more about them than I do. It seems like your tank is too small to house the angle and gourami though.
What is the medication you have for fin-rot? And if it is indeed fin-rot, then the best thing you can do for your fish is keep the water pristine. Do daily 25+% water changes for a week or so and their fins may start healing, this is where the medicine comes in handy. However, most medicines on the market that claim to 'treat' fin-rot actually do not. All they do is speed the process of healing after the infection passes. If you could get some pictures it would be much easier to help you. Also, look into getting an API master liquid test kit, this will make it much easier for you to diagnose certain problems should they arise.
If you have any other questions feel free to post them or PM me. Also, the search engine is your friend, get familiar with using it. Odds are if you have a question, it's already been asked...several times

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