Hi Nancy.
The behavior your betta is displaying is unusual and not common for a betta.
The most common cause for hiding is fear. Sometimes unfavorable water conditions will also cause a fish to hide. So what I will do is touch upon all possibilities with you.
First, the water quality. You're doing well by changing the water every 3-4 days as fresh new water is a fish's best friend. But what I would suggest is avoiding the use of the ph solution as sudden and frequent changes in ph, although minor, can stress a fish.
When changing the water make sure that same temperature water is used and by that I mean when adding the new water it should be close to the same temperature as the water being discarded. Like ph, fish do not take well to sudden changes in water temperature.
As for the fear factor. Too much or too strong of a tank/hood light can frighten a fish thus making them run for cover. Also turbulent water or water that has a lot of current will also send bettas scampering for cover as they are a species of fish who do not appreciate any type of water movement in their environment. This has to do with their natural habitat.
Also if the rest of the container is bare and the "fish house" is the only refuge then the fish will many a time naturally gravitate to the area where it feels most secure so what I would suggest is maybe adding some artificial flowers or other types of ornaments, space permitting, where the fish might not feel so out in the open and vulnerable.
You can even try experimenting by taking the "fish house" out of the container for a period of time to see how the fish reacts.
All in all Nancy I don't feel there's a need to be concerned about the fish's health as from what you have described there doesn't appear to be a disease issue hear.
I would say it's more the bettas instincts at work.
Best wishes and please let me know if you have any questions.
.
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