I've thought about your problem for a couple of days now. It sounds like you are in a welding/fabrication shop and the final inspection is performed at a separate location. Leaks identified by the inspection department are logged and attributed to individual welders. It is then that a witch hunt is initiated and the "bad" welders culled from the herd.

It doesn't sound like a progressive management approach to the problem unless all other avenues have been explored to determine if there is a root cause for the leaks and it has been confirmed that the problem is a welder's skill issue, not a fit-up issue, or selection of the best welding process, filler metal, etc.

You need to use a technique that is fast, environment friendly, and inexpensive to use.

I like the idea of a bubble test if you can apply pressure to each unit and check for leaks using a bubble solution. There is no residue to dispose of. It is fast and the consumables are readily available, easily disposed, and it will require minimal training. One word of caution; limit the air pressure to prevent inadvertent explosive results. You could use a bicycle tire pump to apply the pressure, although it is hard to imagine a welding shop without shop air.

A second choice would be to use a water soluble fluorescent dye that is added to water. The water could be poured into the tank, allow a 15 to 30 minute dwell time to allow the "penetrant" fluid to find it's way through the leakage paths, and a final examination with a black light in an area of subdued lighting (the darker, the better). You can reduce the dwell time by using low pressure air to pressurize the "tank" containing the penetrant fluid. However, if you can apply pressurized air to the tank, I would prefer the bubble test. You would still be handling penetrant materials, but the same penetrant fluid could be recycled several times before it gets so contaminated that it no longer functions properly. I would suggest the penetrant fluid be filtered through some fine mesh screen or simply a large funnel fitted with a disposable milk filter that you can purchase at any farmers supply store. That would take care of any dirt or debris and minimize the cost of performing the test. Still, this method of testing is going to require handling water and the associated mess that can accompany inattentive use of the materials involved. Clean-up would be a simple flushing of the tank with clean water and drying the interior to prevent corrosion.

Penetrant testing is time consuming and there is waste product that has to be disposed. If you use solvent removable dye, you will have paper towel or other wipes to dispose of. If you use water washable penetrant, there will be waste water to dispose of. The time element can approach an hour or so per test if you include preparation, application, dwell time, removal of excess penetrant, and development times. The most costly time constraint will be the final cleaning to remove all the penetrant materials for the welded components. I would not put the penetrant inside the tank due to the difficulty in assuring its complete removal. Liquid penetrants are fluids (oil based) containing concentrated "dye stuffs" (that's the scientific term) that are not easily removed by simple flushing. It gets trapped in all the nooks and crannies often associated with partial penetration groove welds, the roots of fillet welds joints, etc. I've have seen several instances where the test verified the weld soundness, but the parts had to be scrapped due to the inability to remove all the penetrant from the component that was tested. 

Good luck on your project.

Best regards - Al