Inspection:
Tanks should be checked inside and out for rust or corrosion. Secondly, for extreme nicks, chips, dents or other imperfections which could affect function. DOT rated tanks can withstand severe handling without derating. If dented, drilled or severely chipped or gouged they must not be used. All threads must be complete and in good shape.

Cleaning:
All used tanks should be cleaned. If to be used for fuels, the cleaning is not so critical, but must include the removal of any loose particles on the tank interior to prevent liens, filters and injectors from being clogged. Any industrial solvent can be used for cleaning fuel tanks. It should be removed by drying (sun exposure is good) for a couple of days.

For oxidizers, the classic cleaning is a three step operation: First by trichloroethylene, followed by a wash with a high concentration of TSP solution (tri-sodium phosphate), followed by a distilled water rinse and drying. Now since trike is banned for such use, many people are using a hot soapy rinse using Dawn Liquid Detergent followed by a thorough rinse and drying.

Seals:
All seals must be appropriate for the materials to be used in the tanks. Silicone seals will work for almost all fuel type materials and for some oxidizers. Viton is recommended for Nitrous Oxide and eve for LOX stationary seals. Any moving seal in LOX must be Teflon or metal (like a "V" seal ring). Change out all seals in oxidizer valves or regulators for oxidizer proof ones or you will DEFINITELY have a disaster.

Gauges:
Must not be oil filled for oxidizers or have oil contaminant inside the lines or bourdon tube.

Lines:
Again, lines must be compatible. Copper and aluminum and stainless steel will work with nearly all materials (no copper with H2O2!). Make sure the line and fittings are rates for the pressure you are using. Restrain the fittings so the ends will not whip around if they become disconnected or if pressurized while not connected (bad practice). A high pressure gas can drive an unconnected hose or tubing assembly at high velocity, enough to injure or kill.

High Pressure Gas Tanks:
Should be hydrotested to at least twice your anticipated pressure by a certified hydrotester. The danger from ruptured high pressure tanks is so high that you must exert extreme caution and discretion in procuring, conditioning, testing, filling and using these tanks. In using a high pressure tank, it must always incorporate a pressure relief diaphragm or bust disc in the valve assembly so it will vent before bursting (for instance while sitting in the hot desert sun!)

Using Corrosive or Toxic Materials:
This subject will require extreme study of the literature with regard to both safety and legality of use. Suffice it to say that a minimum requirement will include safety suits (possibly with respirators), decontamination equipment and chemicals, spill containment with proper equipment, procedures and personnel, and a disposal plan for unused materials. The best advice is to stay with "safe and sane" propellants of low or no toxicity or corrosiveness.