The snorkeling at Manukan is fantastic! The variety of marine fish that I saw was really astounding. The corals were of great varieties too.
Interestingly, the rarest fish specie that you can here are of the anemonefish! While most of us are used to seeing regular clown anemonefish, being able to spot the large spinecheek anemonefish (Premnas biaculeatus) and orange anemonefish (Amphiprion sandaracinos, somewhat similar to pink anemonefish of A. perideraion) was something that I treasured during the snorkeling trip.
Other than that, you can see colourful damselfish, Moorish idols (Zanclus cornutus), Eastern triangular butterflyfish (Chaetodon baronessa), a few parrotfish, sixbar wrasse (Thalassoma hardwicke), virgate rabbitfish (Siganus virgatus, a big school of them!), vermiculated angelfish (Chaetodontoplus mesoleucus), mirror butterflyfish (Chaetodon speculum), eight-banded butterflyfish, blue-spotted grouper (Cephalopholis cyanostigma), copper-band butterflyfish (Chelmon rostratus), etc.
The corals are definitely healthy and teeming with marine fishes. Among the stony corals you could even spot a number of Bennet's feather of star (Oxycomanthus bennetti) as well as rare bulb tentacle anemones (Entacmaea quadricolor). Otherwise, in term of corals, the usual suspects are colourful staghorn corals, Porites sp. corals, Pavona clavus corals, green mushroom sea anemones (Sarcophyton sp.), blue sea star, bottlebrush Acropora awi corals, various Montipora sp. corals, leaf corals (Pavona decussata), plate corals, knob brain corals (Lobophyllia hemprichii), table corals, etc.