Your source indicated that the early church fathers provide evidence of the fish being added, because Irenaeus, Eusebius and Arnobius don't mention the fish. And it indicates this was because they were not yet inserted into the Greek manuscripts.
Here are the dates for those authors:
Irenaeus 125-202
Eusebius of Caesarea 260-339
Arnobius died 330
Once again, it would be helpful if sources were given for these statements.
In the case of Irenaeus I was able to find one reference, and it is quite clear why he mentions the loaves, but not the fish.
It is from Against Heresies, book II, Chapter XXIV.—Folly of the arguments derived by the heretics from numbers, letters, and syllables.
The reason it doesn't mention to the 2 fishes is because he is making a point about their false understandings based on numbers, in this case the 5 loaves.
ccel.org
4. But that this point is true, that that number which is called five, which agrees in no respect with their argument, and does not harmonize with their system, nor is suitable for a typical manifestation of the things in the Pleroma, [yet has a wide prevalence,] will be proved as follows from the Scriptures. Soter is a name of
five letters; Pater, too, contains five letters; Agape (love), too, consists of five letters; and our Lord, after3173 blessing the five loaves, fed with them five thousand men. Five virgins were called wise by the Lord; and, in like manner, five were styled foolish. Again, five men are said to have been with the Lord when He obtained testimony from the Father,—namely, Peter, and James, and John, and Moses, and Elias.
I found what I think is the reference in Arnobius where he makes brief mention, and
I agree Arnobius doesn't reference the fish. But to say that by his time fish was not in Greek manuscripts does not follow, as we have church fathers from earlier times quoting such.
Arnobius
Against the Heathen
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46. Was He one of us, I say, who by one act of intervention at once healed a hundred or more afflicted with various infirmities and diseases; at whose word only the raging and maddened seas were still, the whirlwinds and tempests were lulled; who walked over the deepest pools with unwet foot; who trod the ridges of the deep, the very waves being astonished, and nature coining under bondage; who with live loaves satisfied five thousand of His followers: and who, lest it might appear to the unbelieving and bard of heart to be an illusion, filled twelve capacious baskets with the fragments that remained?
Here are various church fathers who do reference fish in feeding the multitudes, or eaten with honeycomb, or as a good gift, etc. some at quite early dates.
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Ignatius Died around 108
To the Philippians
And how can He be but God, who raises up the dead, sends away the lame sound of limb, cleanses the lepers, restores sight to the blind, and either increases or transmutes existing substances, as the five loaves and the two fishes, and the water which became wine, and who puts to flight thy whole host by a mere word?
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Justin Martyr 100-165
On the Resurrection
And when they were by every kind of proof persuaded that it was Himself, and in the body, they asked Him to eat with them, that they might thus still more accurately ascertain that He had in verity risen bodily; and He ate honey-comb and fish.
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Origen 185-253
Commentary on Matthew
It must be observed, however, that while in Matthew, Mark, and Luke,5349 the disciples say that they have the five loaves and the two fishes, without indicating whether they were wheaten or of barley, John alone says, that the loaves were barley loaves.
Cyprian 210-258
To Antonianus About Cornelius and Novatian.
23. The Lord also in His Gospel, setting forth the love of God the Father, says, “What man is there of you, whom, if his son ask bread, will he give him a stone? or if he ask a fish, will he give him a serpent? If ye then, being evil, know how to give good gifts unto your children, how much more shall your heavenly Father give good things to them that ask Him?”2494 The Lord is here comparing the father after the flesh, and the eternal and liberal love of God the Father.
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Lactantius 250-325
The Divine Institutes
And when He had tarried there three days, and the people were suffering from hunger, He called His disciples, and asked what quantity of food672 they had with them. But they said that they had five loaves and two fishes in a wallet. Then He commanded that these should be brought forward, and that the multitude, distributed by fifties, should recline on the ground. When the disciples did this, He Himself broke the bread in pieces, and divided the flesh of the fishes, and in His hands both of them were increased.
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Augustine 354-430
Sermon on the Mount, Harmony of the Gospels, Homilies on the Gospels
For after saying, “There is a lad here which hath five barley loaves and two fishes,” he likewise subjoined, “But what are they among so many?” And this last clause really means the same as the expression in question, namely, “except we should go and buy meat for all this people.”
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Constitutions of the Holy Apostles 375-380
He that made Aaron’s dry rod put forth buds,3027 will raise us up in glory; He that raised Him up that had the palsy whole,3028 and healed him that had the withered hand,3029 He that supplied a defective part to him that was born blind from clay and spittle,3030 will raise us up; He that satisfied five thousand men with five loaves and two fishes, and caused a remainder of twelve baskets,3031 and out of water made wine,3032 and sent a piece of money out of a fish’s mouth3033 by me Peter to those that demanded tribute, will raise the dead.
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John Chrysostom 347-407
Homily XLIX.Matt. XIV. 13.
But John saith also, that they were “barley loaves,”1928not mentioning it without object, but teaching us to trample under foot the pride of costly living. Such was the diet of the prophets also.1929
2. “He took therefore the five loaves, and the two fishes, and commanded the multitude,” it is said, “to sit down upon the grass, and looking up to Heaven, He blessed, and brake, and gave to His disciples, and the disciples to the multitude.1930 And they did all eat and were filled, and they took up of the fragments that remained twelve baskets full. And they that had eaten were about five thousand men, beside women and children.”
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