Putting water in an old 300 gallon gas tank

Anyway, like I said, it’s a 300-gallon gas tank. It’s sat unused for about 10 years. I’ve already filled it up with water and let it drain, and it still smells strongly like gas.

Should I not be using this old gas tank for a fire-fighting rig? Is the water that comes out going to be so tainted that it’s going to kill any grass I put it on? Is it going to kill any critters that drink it? Not to be a tree-hugger, but how hazardous is the water going to be when it comes out of this thing?

Probably not a big deal on a homemade rig like that though...

Aside from that I would fill it half way with water and a good grease cutting soap. The soap is the key. Drag the trailer around a while so it will slosh clean then drain. The soap will help the metal release the absorbed gas.

Now about the water you drain.
It will kill grass and any thing else it comes into contact with (fish) if the concentration is strong enough. Now since you drained the gas first (I hope) a few gallons of gas in 300 gallons of water is marginal.
BUT it is strong enough the EPA will drill you a new one on your back side if they found out.
That water is considered Hazardous Waste.
I say this only because I do not want you to take it to the corner car wash and try to use their pressure washer to clean it.