Looked at the video, it has to be catastrophic hose failure, and in terms of can you fill it with plain water, the answer is yes, you would be a fool to use antifreeze with a leak of that magnitude, not would it be unnecessarily harmful to your wallet, but also the environment and any passing animals.
So before doing anything else, check the top hose, and top hose plastic T piece for integrity, the 1800T has a propensity to split along it's seam on the underside.
As it stands, I suspect you will be able to see plainly where the water is coming from.
Best of luck
Brian