as long as I know, in 1982 a famou copywriter 糸井重里first wrote おいしいinstead of 美味しい　for a department poster. It was sensational. Of course people go to that expensive department store can read 美味しいkanji. But, to be able to read kanji was not status simbol anymore. It used be. His copywrite was おいしい生活。We Japanese felt somethig fashonable and modern high class life from that new Japanese. We call it 視覚効果. visual effect. So, novels, CM, PR, MANGA,and even personal letters use hiragana and katakana for visual effect. And sushi restraunts use difficult kanji for visual effect. So, when I write Japanese I use kanji that junior highschool students can read. Because, most Japanese stop studying kanji after passing highschool entrance exam. To sell books about losing weight to many people we use やせる not 痩せる.

Firstable, 池に鯉が有ります。is incorrect Japanese. You must say 池に鯉がいます。 When we use hiragana for word that you can write in kanji there some reasons. For example 鯉is difficult to read for some Japanese like children and young people who are not interested in studying kanji. So, we use hiragana for communicating with those people. We understand that hiragana means carp from the word 池. Another example to use hiragana is that we want to make soft image. 有this kanji means to exist. We feel somethig hard from this kanji. In some sushi restaurants we see many kanji for various fish that many Japanese cant read. We dont care because we can ask the cook and enjyoy communicating with him.

You can use all the forms, because they both right. for using kanji or kana this is a matter of habit. Generally, we use kanji not kana if the world can be write in kanji, some uncommon kanji use ways we will write with kana.^ - ^