Well I was thinking of it.
There is a outdoor place with a fish pond
your goal is to find most rarest 10 goldfish
To get that goal completed you need to breed goldfish
in a fish pond,sell them and use the money to buy supplies
e.g. medicine, water treatment and fish ponds.
You will have staff to help you they will feed,cure and clean the fish pond and their fish.
If you want to look at the fish you click one of the fish pond and it'll zoom in bit like VV overview map
in here the screen will be same as FT screen you can feed,cure and breed the fish yourself
You'll have to breed fish youself because staff only feed,cure,clean and build new fish pond
After checking the fish pond you can zoom back to where you can see all the fish pond and staff you hired.The goldfish should look the same as one in real life.The point of this game is so you can know more about gold fish.
Reason I picked gold fish as topic is because I heard that you can cross breed the gold fish in real life so that must be why we got many kind of goldfish And GFT is supposed to be realistic game and . . . It should run in realtime like FT
Well it's just a idea

good idea!

Here is the limit for GFT
Highest amount of fish in a pond is 20 per pond
Highest amount of pond is 8 that mean you can keep 160 fish
But at beginning you only start with 2 fish pond
Highest amount of staff is 1 per fish pond

Oh one reason you can keep 160 fish because a shopper with not buy just one they'll buy more than one fish

Other reason is the fish will be rather cheap about $3 to $100 roughly but you can change the price a bit.

Here some idea on fish's illness
Ick:least worst fish illness the fish's fed meter slowly drop
Fungus:rather worst fish illness the fish's health meter slowly drop
Fin rot:worstest fish illness in GFT shopper won't want to buy fish that have fin rot

Yes, in real life, one can crossbreed various types of goldfish, but the various types themselves tend to more often come from selective breeding rather than crossbred hybrids.

Goldfish isn't the only type of fish that can hybridize. Rainbowfish will hybridize within their genus, as will some types of Danios. There are three color morphs of tiger barbs, and they will hybridize.

Rainbowfish are actually the more interesting type to hybridize, if Last Day of Work wanted to do this sort of thing. Body types vary from the tiny Threadfin Rainbow to the giant Bosemani Rainbow. Colors range from red to blue to purplish. Many rainbowfish have more than one color. Fins also vary - Threadfin, regular, and twofins are most common. The fish in my avatar is one of my Dwarf Neon Rainbowfish, displaying an elognated twofin.

I think Rainbow fish Tycoon sound good

The fish pond can be replaced with tank to keep rainbow fish in and it is set inside the building

And the fish in RFT still have to be realistic as possible