One of the most critical but hard things to learn is patience! Adding water is one of those.
I suggest that you will get better results when going slower and one way to go is to start the water flow slowly and then as you see what it is doing, you can gradually increase the flow. However, it will be important to fight the urge to speed it up to the point that it blows the sub around.
I find one of the best ways to get my major tank work done is to first perk a cup of coffee to have handy. When I get the urge to rush, I take a good drink of coffee instead.
One of the old posters at work had this to say:
If we don't have time to do the job right, how will we ever find time to do it over? Planted tank folks may need that printed on the back of their hand.
I find tying a rock to the hose is a good way to hold it down until I get back. I even keep one on my siphon.