As
wednesday13
reported, and I concur, wels are not psychologically stable and robust fish, it seems. Any change, any stress, any significant event in their tank / pond, sends them sulking for long times, even such an innocent change as changing one species of frozen-fortified fish for another, say mullet for a mackerel.
Mine too goes on frequent fasts, sometimes days, sometimes weeks, rarely 1-2 months, mind you, he is clearly the alpha of his 25K, no one messes with him. I think in your pond, him not being the alpha at all and with so many rivals, it can be explainable that his appetite is relatively lower, but I can't say it is lower than average on the absolute scale, perhaps mine is still managing to feed more than average, who knows?