How Do Fish Have Sex?
this depends on the species of fish.most are open spawners,female releases eggs and the male fertilises them,the developing fry are left to their own devices.as mentioned some fish give birth to live fry,platies and guppies are such examples.then there are the cichlids[sick-lids],the eggs are spawned,and the parents defend the eggs and fry until they are free swimming.then you have guoramis,siamese fighters and some catfish,which build bubble nests,to lay eggs.
practical fishkeeping magazine
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Most fish are oviparous: females lay eggs which are fertilised externally by the male. But some fish are viviparous. Viviparous fish mate in the usual way, the eggs are fertilised, gestated and hatch inside the female who then gives birth to live young.