changed my betta's tank completely. It's bigger, has
more plants, and is just very different. Could such a
change result in a brief "depression stage"?
A: Hello Sam :). Well first off, I want to say "Thank You" on behalf of your betta, for taking such good care of him. In my experience, a larger tank with better environment can in the end only help your betta's health and spirit. Will he go through a transition period? Probably. He is after all, being introduced to a new surrounding, which to him will be unfamiliar and may seem (at first) somewhat threatening. Will he go into depression? Not necessarily.
I bet you that bettas, in their little heads, think, when are first released in a new tank: "Oh, cool, it's big! But are there any predators in it?". So even if they liked the water, filter, plants etc, they may still act a bit cautious at first. Soon they roam around long enough to realize nothing is hidden behind the intake tube or the rock, or plant and they figure out that they are safe, and act accordingly by taking over the tank and flourishing :).
Now let's talk about betta depression (wait, let me put my betta psychologist hat on LOL). Dr. Faith says: Betta depression is real. Bettas do go into a depression phase which can last from a few days to a couple weeks when:
If your new betta is sulking, it may last from a few days to long weeks :((. Extreme cases of depression will involve starvation, meaning Mr. Betta will go on a hunger strike and not eat a nada. This can cause some problem as it weakens the fish and may cause it, on the very long run, to perish.
What to do when your betta is depressed and sulking:
In closing, bettas
rarely sulk when they are moved from one tank to
another within the same home because the water
parameters and life style remain about the same. And
for as long as you don't take a betta out of a 20
gal tank to stick it in a small jar, bettas will
usually be pretty forgiving of your reorganizing
their homes. So I think your fish will be just fine
and should actually be more active and happy now
than before :). Hope this helped you.