Find out the optimum temperature range for the species, then incubate the embryos in that temperature range under flow-through system. This gonna improve the hatching percentage.

Would be useful to know what kind of fish you are breeding. But here are 3 links to how to breed Angelfish and African and Amercian cichlids. These should give you a good idea how to proceed.

http://www.tropicalfishcentre.co.uk/breedingangels.htm

https://www.researchgate.net/post/Is_there_any_one_with_working_protocol_for_hatching_carnivorous_small_egg-sized_fish?_ec=topicPostOverviewQuestionRecommendations

https://www.youtube.com/watch?v=lnjD04mFEjY

Tomas

It would be important to know which species are you interested in. Regards.

The species am working with is Lates. It produces several small-size eggs. we have managed to fertilize them under artificial conditions successfully. However, incubation takes such a long time and the fertilized eggs eventually get infested with fungus. Any body who has worked with this scenario can guide

Eggs in Lates calcarifer develop rapidly compared to other fish species. I suggest you to read the document by FAO from http://www.fao.org/tempref/FI/CDrom/aquaculture/I1129m/file/en/en_barramundi.htm, where it is reported that "Fertilised eggs undergo rapid development and hatching occurs 12–17 hours after fertilisation at 27–30 °C". You might have a problem with water quality, water renewal or temperature level. I hope it can help.