Puzzle 019: The Fish Tank

ナゾ０１９　水槽の魚
私は水槽に１匹魚を買っており、
ジョンと名付けている。その水槽には、
ときどきジョン以外の魚を入れるが
しばらく放って置くと後から入れた
魚は姿を消してしまう。ところで、
ジョンに以外で水槽に入れる魚は共通
の呼び方をしているのだが、それは
何だと思うかい？
ひらがな２文字で答えてね。

Google Translate
I will buy a fish tank and one fish are named John. The tank that the fish and put him alone for a while after I put the put the other fish would disappear sometimes John. By the way, put the fish tank but other than that to John prefer to be called common, is that a Guess what? I answered two hiragana characters.

Notes on Puzzle
You have to figure out what happens to fish when they get put in John's tank, aka why do they disappear? The answer (spelled out with 2 hiragana) is kind of sad.

After you solve this puzzle, you get the a pet fish! I named my fish “Nemo”. You also get access to the Fish minigame, which involved changing one 2-hiragana word into another. You give the hiragana to a fish, who eats it and spits out a new set of hiragana. At the top is a hiragana chart; the fish change the hiragana based on the chart. A fish can either switch the first and second hiragana, move the hiragana horizontally on the chart or vertically on the chart. I have no idea how this game would translate to English, which is alphabetic. Perhaps create a limited set of syllables and chart them? Hmm, it would be a bit tricky for sure.