Aether and Entropy work together to create randomness sometimes alright. See PU on mutants. Fate Egg and RT on Skeletons are other examples of non-Entropy randomness.
However, this has deeper thematic roots than that. Aetherfish is purely aetherial and the entire process, from summoning him to using his abilities, is fueled completely by aetherial energies. There's no way to justify the use of entropic -quanta- even if you feel like randomness belongs to Entropy (which isn't entirely true either). His ability is only random because of his own incompetence. If he could control himself, he'd be Lightning or PU on a stick. In other words, OP. This is, of course, irrelevant because the card bends to the lore, so there's no way he'd be smart enough to control the powers he's barely even aware of.
My early draft of aetherfish was only able to do Lightning, PU, Quint and Lobo, but the truth is that aetherfish has all kinds of aetherial energies in him. He embodies everything Aether is capable of and all of its potential, so he has to be able to do anything aetherial.