Next to a living room tank, I try to relax (not hard in an old family rocker) and not move much. Even feeding, which may be from an occasional baster squirt of defrosted insect larvae, is done as leisurely and unobtrusively as possible. They obviously know that that "big face" or whatever they think, is around, But their behavior is different without too a obvious (or over shadowing) human presence. I have been distressed to observe a couple of shy fish that will not even come out to feed (obviously not livebearers) when a person is close to the aquarium.

You raise the question of oxygen and it is to you credit that that is on your "check list" of things to look for. The top cm or so of water will have a slightly higher level of oxygen in it. In crowded tanks and on hot days, it is not unusual to see more fish at the surface than usual.

If it is an oxygen problem, they will stay at the surface when you are not around, even if the lights go off and it is dark.. If the problem gets worse they may settle to the tank bottom. (This is not to be confused with sleeping at night.) If they discolor or their breathing continues to slow down, they are indeed suffering from nitrate poisoning and need help very quickly before they suffocate.

Try sitting quietly near the tank (bring a good book or magazine) and then look to see if they are more dispersed through out the tank. If they are, all is well.

Is that one male guppy, Shadow, smaller (or is his mass less) than the other guppies? He may not need as much O2.

If they are still at the surface, the following are things I would do or at least consider:
When convenient, do an extra partial water change, but please do let the water sit some after treating it and warm to tank temperature before adding it. I would probably run a gravel vacuum on the bottom and look for low spots or corners where oxygen-absorbing detritus might have gathered. Do be sensitive to the smell of rotten eggs, a.k.a. that very toxic hydrogen sulfate, which can develop in odd, out of the way places in the gravel.

I would check to see that whatever filter I had was not clogged with debris. The power filter or box filter floss would still look gray, even after rinsing. The sponge filter would not be all that light colored, after some squeezing under the faucet (we are lucky to not have chloramines and only have lightly chlorinated water). If it didn't bother the guppies, I would turn up the stream of bubbled on any box or sponge filter. If the power filter cartridge or pad was hopelessly clogged, I might remove it and just let the filter run as is to roil the surface some for a day or two. Maybe an airstone would be added to the tank. If I had a power head (either on a big sponge or attached to an UG filter) I would aim that "rudder" or flap by the outflow more towards the water's surface.

If I suspected that (or tested and discovered that...) ammonia and other wastes were too numerous after the water change, I probably would put a Polyfilter is a filter box to absorb the extra nitrogenous "stuff". You might prefer using a bag of rinsed activated carbon. We need to remember that the Polyfilter will turn from off-white to black and be used up. So too the activated carbon, on an organically rich aquarium, will be done probably within a week (two weeks of "normal" use.)

All of these things are routine maintenance activities, but that is what keeps the system going. (Just back from an auto care place, I am reminded that this is true in a number of facets of life.)

Less a part of the routine, you might look to see if there is a dead fish somewhere. Their decay can consume quite a bit of oxygen. Also, if I had plants in the tank, I would extend the light period a couple of hours a day. That would increase their oxygen production and decrease the hours when the plants actually absorb some oxygen in the dark.

That is still tank care. I'm sure the list is not exhaustive. Others may wish to add their ideas. And you probably have already done/ are doing a lot of these things, This doesn't include really desperate measures like taking an eye dropper and adding 1 drop per gallon of hydrogen peroxide. (And I have the darndest time doing that because the fish swim towards where the drops hit the water. Perhaps adding a drop every couple of minutes or every several second to the power filter outflow until the drop per gallon limit is reached would be a better approach.) But that most likely is not something you will have to fool with.

If a hobbyist keeps fish long enough or expands to too many aquariums, they will see this problem. The loss of s treasured fish in a neglected tank is a bitter experience which too many of us have experienced. I wouldn't wish it upon you. :)