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The app focuses on a single Kanji letter. All letters you practice basically mean fish though; there are many Kanji letters that mean fish since there are a variety of fishes on the Earth, such as sardine, bonito, salmon, mackerel, tuna and so on.
Here is a tip that hopefully would help you to get to used to learn Kanji letter. Some Kanji letters consist of basic Kanji letters. As I mentioned "Some", This rule does not always fall into all Kanji letters but you may get an idea. Here is an example. There is a Kanji letter that means a parent. This Kanji letter consists of three basic Kanji letters, such as meaning of "look" and "stand" and "tree". You might wonder why "parent" relates to "look" and "stand" and "tree". As far as I know, a parent is supposed to look a children standing on the tree, that is why. In the same manner, Kanji letters you are going to practice on the JP Fish app include a basic Kanji letter meaning of fish.
This is a basic Kanji letter meaning of fish.
[魚]
Here are a few example that I features in this app.
This is a Kanji letter meaning of a cod.
[鱈]
As you see, this letter includes a fish letter on the left side. On the right side Kanji letter means "snow". I am not really sure why a letter of "snow" is included though; it is possibly based on the fact that we tend to eat cod fish during autumn and winter in Japan or a cod fish lives in cold region.
This is a Kanji letter meaning of Spanish mackerel.
[鰆]
As you see, this letter also includes a fish letter on the left side. On the right side Kanji letter means "Spring". In this case, it may be based on the fact that we tend to eat it during Spring in Japan.
This is a Kanji letter meaning of a sole.
[鮃]
As you see, this letter includes a fish letter on the left side as well. On the right side Kanji letter means "flat". As you imagine, it would relate to a shape of the fish, I suppose.
I wish I could explain all Kanji letters though; I am afraid that I am not a professional person. I hope that you would get an idea about how Kanji letters are made and you may interest in some letters in this app. Perhaps, some letters you practice in the app might come in handy, when you go to a sushi bar in Japan someday.
Enjoy writing Japanese letters!