Are you getting confused between the different requirements and which ones are satisfied by the Sepia Officinalis, Murex Trunculus and Janthina?
This table compares the three most prominent candidates for the chilazon. Ideally, a grading system would be preferable to a yes/no response, but feel free to take the list of requirements and make your own grading based on your research.
|Feature||Sepia Officinalis||Murex Trunculus||Janthina|
|Body has the form of a fish||✓||✗||✗|
|Body color resembles the sea||✓||?||✗|
|Comes up every 70 years||✓||✗||✓|
|Caught with nets||✓||✗||✓|
|Tentacles bent like hooks (Mishnah in Keilim)||✓||✗||✗|
|Treatment for hemorrhoids||✓||✗||✗|
|Hides in the sand||✓||?||✗|
|Dye is better while chilazon is alive||✓||?||✗|
|Shell grows with it||✓||✓||✓|
|Blood is black (Rambam)||✓||✗||✗|
|Passes the k’la ilan test||✓||✗||?|
|Chemically differs from Indigo||✓||✗||?|
|Jumps about when caught hastening its death (Tosafot Shabbat 75a)||✓||✗||✗|