Hard to tell species with sunfish so many are hybrids, most familiar hybrids are bluegill (Lepomis macrochirus) x green sunfish (L. cyanellus) and bluegill x redear sunfish (L. microlophus) crosses. These hybrids occur naturally where both species occupy the same habitat, and they are produced by some hatcheries.
But yep I would guess it has at least some Pumpkinseed sunfish (Lepomis gibbosus) in it.
Just to add to the confusion....
"Ã¢â‚¬Å"a number of different kinds of hybrid sunfishes . . are not sterile, are fully capable of producing abundant
F2 and Fs generations, and can be successfully backcrossed to parent species and even outcrossed to nonparental speciesÃ¢â‚¬Â
Let's start with the fact that that paper is from 1984, and even that only claims a 2% probability of F1 hybridization within two measured environments in Georgia. I think the word is "insignificant", and they used it, not me.
The Lepomis cyanellus picture on the NANFA site has a white background, making it impossible to see the white edges on the fins. This is a purely cyanellus characteristic. The photo of a macrochirus x cyanellus is mislabeled, it is macrochirus x gibbosus. You will note that the hybrid gibbosus x cyanellus shows the bright pumpkin orange breast of gibbosus, OP's picture has no such color. This is not a variable character, it is one that always happens with hybrids between these species, both sexes.
The elongate body eliminates hybridization with macrochirus, gibbosus, and most of the other common Sunfish species. The listed species and most of the other common species have a deep body shape. The body shape of a hybrid would be more intermediate in height to length ratio, and it is not. The other elongate species, the Warmouth, has a bigger mouth than a green. The mouth is difficult to see in OP's photo, and does appear to be small. However, That could be deceptive, as the length of the mouth is concealed by a strong light reflection in the glass.
Since the OP is in California, the fish was either an introduced population or an accidental import from elsewhere. Since he mistook it for an African Cichlid, I'm guessing that he found it in a store. Therefore the source is indeterminable. That gives us, at WORST a 98% probability that it is a pure Lepomis cyanellus, thus my identification of a species that I've caught in 5 different states. How many have you caught near London, James? :roll: