Okay, I feel like an idiot but I can't figure out how to sell my fish. I put them in the isolation tank and click on sell fish and then when I go to the store there are no fish in there to be sold. I just can't figure it out! Can anybody help me with simple instructions? Thanks in advance!

That happens more than you'd think! The isolation tank is used for breeding and examining fish. To sell fish, you need to put them in the selling tank (either by dragging them to the tab that has a "$" on it, or by putting them into the isolation tank, switching to the selling tank ($), and then taking them out of the iso tank). You can use the selling tenk for breeding and growing fish, but keep in mind that any adult fish that are in the tank when you tap the Sell Fish button will be for sale.

Thanks so much, Barbara! I knew it must be something simple that I hadn't noticed. I thought the $ tank was Tank #3 and didn't look closely enough at the symbol. My store is operating now, thanks again.

Anytime! I'm always glad to help!

Here's a weird one for you. Fish in iso tank does not appear when switching to shop view. Dragging to tank $ (cunningly disguised as tank 3) does work.

There's nothing weird about that. That's how the game works. The little "tank" you see in the shop is used to display a fish that has just been sold. The iso tank displays only when you're looking at one of the three fish tanks. The selling tank (Tank $) can be used for breeding and growing fish, just like the other two tanks. The only (and important) difference is that any adult fish in that tank when you switch to the shop (by tapping the Sell Fish button) will be for sale. You could inadvertently sell fish that you want to keep unless you are aware of the distinction.

You might want to check out this page - it explains a lot of the basics of Fish Tycoon.

how and were should i sell my koi fish.

If you can find a koi in Fish Tycoon, then you can sell it in the manner described above. If not, perhaps a visit to your local pet store would be a good idea!