I use RO water in my tanks and collect it in 5 gallon containers for future use...how long can I store it safely before it goes stagnant and is no longer safe to use ?

Don't hold me to this but I would imagine if the water is open to the elements, it will go bad quicker than if it is kept in sealed containers. If it is kept in a sealed container it should be okay for quite a long time. However, I'm not a water expert so somebody else may chime in with some more advice for you

after collecting from the refilling station and sealed,
it will still be good for a year. however, if exposed to
sunlight for quite sometime, the plastic container itself
contaminate the water... bad for human consumption, but
still be fine for water change of your tank.

Sounds good guys...the containers are meant for water and are sealed..and not kept near any direct sunlight...so it looks like I'm good to go...thanks for the answers guys...as always you were a big help.