Get a bigger tank for the skink. Otherwise, rehome the skink.
I think it’s both cost and to many animals. They want to just buy a standard glass of enclosure but I would like to build a custom wood oneI think it depends upon your parents' motivation for the ultimatum they gave you.
If they simply want you to save money, I would build a nice big custom enclosure for the skink, which you can do quite inexpensively. Actually, I love DIY projects so I'd probably go this route no matter what. You can maximize whatever space you have and design it specifically for the skink, and hopefully keep your fish.
If they just want to save space...well, again, a DIY enclosure could be made that is incorporated into a stand that also holds one of your other tanks. Does the 125 have a sump? If not, build a skink enclosure into the stand beneath the tank. Or, if the 125 is not sitting too high up, maybe build the enclosure so that is mounted above the tank, making sure to leave lots of room for maintenance access. Since the skink doesn't really need height, you can make a nice big enclosure that is still not too tall so this might work, and it won't eat up any more real estate than you are now utilizing.
Of course, if they have just decided you have too many critters...sorry, bud, you're hooped.![]()