Whenever I look at hatchery pictures the pros make their tanks look
immaculate. OK understandably. But not just free of detritus, also
free of algae. And sterile looking, without plants or any features.

I'm experimenting with breeding now. I thought I'd try using direct
lighting in a bare bottom grow out tank and allow algae to thrive. I
grew wonderful sheets of Nitrate sucking algae, and thought, hey, this
is a good thing, right? Fry from the same spawn that where in an
algae free tank and received the same diet seem to not have nearly as
deep colors. So I thought, why don't the pros use algae for diet and
supplemental filtration?

Then recently I started seeing bulges under parts of the algae sheets.
Not knowing what gas was under it I broke the algae sheet with a
siphon and sucked the gas out. Is this one of the reasons they don't
bother with it? Why wouldn't you use algae or certain nutrient-sponge
plants in fry grow-out tanks?