Hi and welcome to the forum
It's a bit hard to tell if the fish is healthy due to the dark picture and the angle of the picture. However, it didn't appear (from what I could tell) to be ill.
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The main reason fish grow slowly is lack of adequate food, low temperature and lack of water changes.
Baby fish and young fish should be fed 3-5 times per day with a variety of foods including dry, frozen and live foods. Marine mix is a combination of prawn, fish and squid, and is an excellent food for rearing up angelfish. If you can't find marine mix, buy a bag of frozen prawns/ shrimp from a fishing shop or supermarket and keep them in the freezer. Take one prawn out and defrost it. Remove the head, shell and gut (thin black tube in body) and throw these bits away. Use a pair of scissors to cut the remaining prawn tail into small pieces and offer 1 or 2 bits at a time. Feed until all the fish are full.
Feed a bit of dry food first and then frozen (but defrosted) and then live food if you want. Live food can be mozzie larvae, daphnia, brineshrimp, ants, ant eggs, aphids, fruit flies, or most other small non toxic insects that haven't been gassed with anything.
Frozen foods can include prawn, fish, squid/ octopus, bloodworms (however use a reputable brand and try to get ones that have been irradiated, because some brands can cause problems). Mysis shrimp, krill, brineshrimp, daphnia are also suitable frozen foods.
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The best temperature for rearing young tropical aquarium fish is 28C. However, 25C is fine for your fish, which aren't babies.
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If you do a big water change several times a week, it will help them grow faster. Water changes dilute nutrients and hormones released by the growing fish and this can help them get bigger, quicker.
In a big tank with not many fish, a 75% water change once a week will help them grow.
