The only way to have a redtail cat happy and healthy long-term is a thousands gallon pond, inside - people do it in their basement - but you need to do a lot of research first.
Doing a "small one first" is fine, but only if you intend to build a bigger one, as the fish outgrows it. The bigger the catfish gets, the harder you will be able to give it away.
+1 I would say anything above 6 inches will be getting harder to give away people get them small so they can expirence growing them out etc...
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