From 1 of the threads referenced above;

" In Role of Male Parental Care in Survival of Larval Bluegills by Mark B. Bain and Louis A. Helfrich in Transactions of the American Fisheries Society 112: 47-52, 1983 the authors noted optimum Bluegill nest materials were exposed coarse gravel ( 8-32 mm diameter) and pebbles (32-64 mm) in nest substrate and that Bluegill removed particles smaller than 2 mm. Particles larger than 8 mm provided suitable interstitial space to accommodate bluegill larvae. Survival of larvae was directly correlated with the proportion of coarse substrate in the nest."