One common mistake made by novice classroom salmon aquaculturists is using gravel that is too small. Individual cobbles should be a minimum of one inch in diameter. Smaller cobbles don't provide the "hiding places" where newly hatched alevin like to congregate. More importantly, decomposing organic debris (dead eggs/fish, fish waste and fish food) can accumulate under the smaller sized gravel - usually undetected - giving rise to dangerous fungal and bacterial growth. Gravel of appropriate size cannot generally be found in a fish or aquarium store. Rather, it is more likely to be found at gravel pits, landscape supply stores, or home centers. A creative approach employed by some teachers is to assign students to bring several rocks from home. This type of approach generally yields all the gravel that is needed because the gravel layer in the incubator should only be one rock deep.