The tanks should have water in them normally. The outlets in the tanks sit up near the top, like speedbump said, about a foot down from the top of the tank. Water will stay in the tank up to the bottom of the outlet. When it rises up higher, it drains out. Not sure if you're able to see it, but the final outlet on the second tank should have a baffle of some kind, an elbow or a tee pipe and not a straight opening into the pipe. The baffle helps prevent solids from exiting, and the baffle may or may not have a filter device.
I don't know what hydrojetting a septic system entails. Where I live they just pump the tanks out and leave it at that. The waste haulers dump what they pump at a wastewater treatment facility (it's not legal to dump it anywhere else, except back in the septic tank, but that kind of defeats the purpose of pumping in the first place). Also, no one should be adding chemicals to a septic system, including pouring stuff down the drain in the home. If the tanks aren't overloaded or backing up, then just leave them alone, they sound fine.
If any tank needs to be pumped it would be the first tank as that is where all the solids are going to be building up (though some solids can build up in the second tank, too, but not nearly as much). The reason you want to prevent the solid level from getting too high is it reduces the volume of water the tank can hold and decreases the holding time of the water, which means it will travel through the tank faster. The faster the water moves, the more solids it will keep afloat and carry into the next tank and possibly out into the drainfield. Inside a septic tank should be still and motionless, not turbulent and flowing.