It is pure RNG.

The way reduction works is that you will always receive a certain amount of crystals, plus a secondary item that is picked randomly based on your Purity. The actual yield of the items is not based on Purity, but the level of the collectible being reduced.

Typically, reducible collectibles need to have 2/8 or higher Purity for clusters to have a chance at showing up as the secondary item. At 2/8 this chance is 100%, but higher Purity scores will split this chance up with eligible Aethersand drops.

Lv.56 Fish: Dravanian Bass, Manasail (Least..)
Lv.58 Fish: Caiman, Pteranodon (More)
Lv.60 Fish: Thaliak Caiman, Tupuxuara (Most!)

The only thing you're ever guaranteed to receive from reduction is crystals. FSH lacks the ability to select a specific purity range like BTN and MIN, so if you absolutely need to get Ice/Water Clusters, it's best to pay a visit to Mor Dhona.

For anyone who has yet to fish up the collectible fish, click here! (Link)