Oh ok. Looks like he's healthy to me. I get what you mean now.

I had a female betta that grew like crazy after I got her at the pet store. She was maybe an inch when i got her, and she died at nearly 2". Yours should grow as well. Size has nothing to do with age. He was probably just not treated right at the pet store. You can stick with your 5 day water changes, but I would change a little of it say every couple days (maybe a cup) to help the nitrites stay lower (should be 0).

Here's the link to the article:
Cycling process

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Phishie
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"An ounce of prevention is worth a pound of cure."