The usual reason for starting aeration slowly is that your pond may be anaerobic at the bottom. The decay and other microbial products produced in an anaerobic environment can be quite toxic (like in a septic tank). Aeration brings this stuff up into the aerobic layer where your fish live, so you want to bring it up a little at a time while you are simultaneously making the lower environment more aerobic. The deeper your pond, the more dead organic material at the bottom, and the warmer the water, the more likely the bottom will be anaerobic and toxic.
