Personally, I think stomach pumps should be the last thing any self respecting fly shop or on line fly fishing resource should offer to the fly fisher. Oh, the sampling can be most enlightening, but more often than not (no I don’t have any statistical data) I would imagine the device is misused and causes harm to the fish.
I mean just look at that stillwater sampling of mega chironomids, damsel fly and mayfly nymphs. How much easier now to tie on the correct size and color of an imitation. But, seriously, you want it that easy? While potentially doing harm to the fish? I will say this is one thing (the only thing probably) in which, I think Brian Chan errors. A fishery biologist, such as he, knows how to use a simple, crude device as a stomach pump and has a theoretical need to study food samples from fish and the health of a lake or river. The rest of us can study up and forgo the pump. I don’t believe I have seen a presentation by Mr. Chan in which the pump is not presented and demonstrated at least on lakes. He takes great care to use cradles to land fish and is obviously respectful of the fish. Others, I am not so sure of.
Is there available written data on the hatches/aquatic life of the body of water you intend to fish? What patterns imitate those food sources? Where are they likely living, emerging, drifting, etc. in that lake or river? At what time of day do they provide the best food source for the fish? What months are they best available? What do other fly fishers tell you? What techniques are you seeing successfully used and where on the body of water?
When you get to this body of water, what do you see? Are there visible hatches? Are birds feeding above the water? How are the rise forms of the fish (sub surface slashes, porpoising, sips, engulfing wallops, airborne projectiles)? What do you see on the water’s surface, nearby vegetation, on the rocks? What is possibly protruding from the fish’s mouth you are about to release.
Stomach pumps may provide that extra reassurance of what to use, but given the probable harm you will cause (if catching and releasing), forgo the pump and use your brain and power of observation more often. Unless you are Brian Chan and/or a fishery’s biologist?