That's good you have well water, so that's likely not what's causing your current issues (though still good to use the Arowana Guard in case your well has some impurities or heavy metals).
However, putting water from an old tank into a new tank does absolutely nothing for your cycle/establishing beneficial bacteria -- you need to bring over some established bio-media from the old tank's filter, or a sponge filter or even some decorations or gravel.
So I'm guessing that your tank isn't adequately cycled and ammonia/nitrite levels could be fluctuating, causing the problems/stress you're seeing.
Would suggest to do frequent water changes (adding some salt as kno4te advises) -- and some way or another (online?) get a water test kit ASAP, it's vital esp. in these situations.
One more thing: I personally wouldn't turn on the UV until after your cycle is better established and the beneficial bacteria are more properly seeded and colonized on bio media and tank surfaces.
