1: ἰχθύς
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(Strong's #2486 — Noun Masculine — ichthus — ikh-thoos' )

denotes "a fish," Matthew 7:10; Mark 6:38 , etc.; apart from the Gospels, only in 1—Corinthians 15:39 .

2: ἰχθύδιον
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(Strong's #2485 — Noun Neuter — ichthudion — ikh-thoo'-dee-on )

is a diminutive of No. 1, "a little fish," Matthew 15:34; Mark 8:7 .

3: ὀψάριον
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(Strong's #3795 — Noun Neuter — opsarion — op-sar'-ee-on )

is a diminutive of opson, "cooked meat," or "a relish, a dainty dish, especially of fish;" it denotes "a little fish," John 6:9,11; 21:9,10,13 .