My experience with firemouths has been that they are slow growers. Turn the temp to about 82, add some beefheart to the diet. Will help it grow faster if thats what you want. Here is some info:
Firemouth - This fish is now classified as "Thorichtys Meeki"
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Habitat
Bottom dwelling cichlid. From Rivers in Mexico and Guatemala
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Diet
Likes its food to be mixed up a bit. Such as, flakes one day, shrimp or blood worms the next.. etc
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Length
Males may grow up to 6 inches (15cm).
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Sexing/Breeding
Egg Layer - Males have longer, flowing fins, and tend to be more colorful.
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Aggression
This fish tends to have a "live and let live" attitude to it. However, it can be quite the bully when housed with a fish it doesn't like. This fish is quite capable of taking up for itself without becoming hyper dominant.
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Life Span
In the right conditions, Firemouth cichlids
can live up to 15 years.
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Care
Ph-7.0 -
Hardness 9/10 dgh -
Temp. around 75f