I'm not really an expert on tank less water heaters, but a couple of things I have picked up from this forum is first, tank less heaters require mega amounts of power be it electricity or gas. Most homes do not have heavy enough supplies of either. Secondly, LPG is quite expensive compared to natural gas. I suspect another tank would be your best option, and probably not a "smaller" one. Another 50 gallon tank would not be much more expensive than a smaller one (can't quote prices) I would expect some of the pros to jump in on this thread with better and more complete advice that this, so hand on for a couple of days or so.