Inspecting water for fish

SimsMegaDeluxe​  The "unidentified" fish are those with a level higher than your sim's skill level.  For example, deathfish will be listed as "unknown fish" until your sim hits level 10.

What I don't know is whether anything else can reveal the fish early.  But it's easy enough to test.

SimsMegaDeluxe​ ... 'Unidentified Fish' are fish you have not caught yet and theoretically, your Sim doesn't know exist.

As far as what fish your Sims can catch ... The "Rules" are not as binding as the game suggests. In fact ... If your Sim knows that alley catfish can be used for bait to catch angelfish ... Or that angelfish can be used for bait to catch deathfish ... Your Sim can catch angelfish and deathfish as soon as they use the proper bait and it doesn't depend on what level they are.

However ... Catching the proper fish when using the proper bait is never a guarantee in the first place ... But attempting to catch higher level fish and being successful in doing so ... Will increase your Sim's levels much quicker. The information given is not a 'spoiler' though ... Because your Sim can purchase books regarding fish and their desired bait at the bookstore.

Another interesting thing about your Sims and Fishing ... The quality of fish your Sim catches increases, the more fish of any particular type your Sim is catching ... As does the value. The value does not consistently rise with each fish but increases in overall value gradually.

If you have 10 fish of the same type in your Sim's Inventory ... Your Sim can Stock a Pond. Ponds are usually found in yards or parks and are Freshwater ... But it doesn't matter, you can still stock Saltwater fish in a Freshwater Pond in the game. This even includes deathfish ... Which can become quite lucrative when your Sim has been catching a lot of them and they increase in value... And you can catch them any time day or night in a stocked pond. Stocking a pond multiple times with the same type fish, will increase the rate at which you catch those fish, but still will not create an 'active' spot (where they are jumping out of the water).

Last but not least ... After reaching a certain level in cooking ... Your Sim can make Sushi for Dinner (5 pm until Breakfast). If your Sim has fish in their inventory or the fridge, Sushi won't cost anything to make and does not take long to prepare (big time saver).

HotBlackSand​  I always see the fish properly named when my sim has the required skill level even if the sim hasn't caught any of that type yet.  I think I remember seeing the fish appear on the list once my sim had caught one even if they didn't have a high-enough level yet, but I can't be sure because it would have been a long time ago.  These days, I only have them try with the proper bait once they're at the relevant skill level, and they can't catch fish with the wrong bait (or none) until they reach that level.

Thanks puzzlezaddict​ ... I tend to stock the pond in City Park in a spot directly across the road from the Grocery Store with alley catfish, angelfish and a deathfish. That way any Sim I desire to learn the fishing skill can go to the Grocery Store and buy cheese ... Then knock out the two active spots in the City Park Pond, gaining enough skill to use bait. Then I have them use cheese to catch alley catfish until they can use live bait ... Then alley catfish to catch angelfish until they have about five ... Then angelfish to catch deathfish.

They can usually get to level 10 that way over a weekend without too much effort and start catching deathfish around level 7 or 8.

That way the Sims can also hang out in the center of town and introduce themselves to a lot of people ... And then they tend to stock the ponds in their yards the same way ... Unless I just prefer they have a Koi Pond instead.