Unlike with .177 & 22 cals, there are no .25 cal lead pellets available that were really light. The lightest on the market until recently was the FTT 19.91 gr., and the same pellet under the Hatsan name. Now, even the FTT's are 20.06 gr. minimum. With .25 cals, the problem is more like lack of lower weight options.
At 16.54 gr., lead-free Exacts are the lightest .25 cals around, AFAIK. Shooting those from my H135 felt almost like dry-firing: slammy shot cycle, heavy recoil and terrible noise. 20-grain FTT's were better, but not much. With 25 - 30 grain pellets the gun worked as it should.
Around 24 - 25 grains seems to be the best pellet weight for .25 cal springers as far as mainspring longevity is concerned. However, it may be that your 125 shoots some pellet not in that range most accurately, and, unless that most accurate pellet has lousy BC, meaning unethical terminal performance for long range hunting shots, it's best to use that.