The story of Genesis indicates in its pages that within centuries of being created humanity as a whole became so evil they did not deserve to live. God agreed with this position and so as the righteous judge properly and lawfully sentenced humanity to be annihilated. In respect of the rest of the creatures since God owned all of it by virtue of his creative rights, he determined that in order to properly sentence humanity, he would destroy all. So God destroyed everything, every person, every creature.
Yet God also provided a way forward for humanity. Hence he decided to demonstrate grace towards Noah, his family and either two or seven of every kind of creature, depending upon whether they were clean or unclean and in order for humanity to have a second chance. God knew that humanity did not deserve it. Yet, out of his kindness and mercy, he commanded Noah to build an Ark in which his family and all animals - could enter and be safe. Nowhere in any of the passages does it indicate he took fish into the Ark.
The question has been raised - how could Noah fit all of the world's animals in to the Ark? And most skepticism has arisen around the quantity and size of the animals. Not the only skepticism of course. How big was the ark, how did they all get there? Etc. Yet this topic is only about size of the animals - nothing else.
The skepticism seems to arise that due to the very large size of some adult animals in the world, that this proves - ipso facto it could not have happened. Yet the contrary position that the animals or many of the animals or even some of the animals may be infants interestingly changes the dynamics of the skeptic's queries. And this obviously is something that most of them had even considered. I think that is what hurts most. The fact that it is so obvious and yet they still missed it.
Some suggest it defies logic to put infants into the ark. I say it defies logic not to consider the possibility. After all, you don't want adult man eaters in the ark. You don't want adult dangerous animals in the Ark. You don't want adult animals that will clearly take up far too much room in the ark. You want animals that are small, young, fit, and healthy, but not ferocious man eaters that will cause not only havoc for the humans but the other animals as well. Common sense would suggest young and infants would be the most likely candidates.
Now some skeptics suggest that this is preposterous. They suggest that animal infants without full grown mothers will lack proper child -rearing regiments. Of course - there may be some truth to this. It certainly would be ideal if all infants had mothers to mother them and train them. Yet it certainly is not impossible that such infants would not survive without their mother, especially if they are being watched and cared for by others. In fact, there are many stories of wild infants surviving into adult hood without ANY MOTHER whatsoever helping them. But hey, let's not confuse ourselves with the facts.
As for the necessity to teach how to survive in the environment - the story is that after the flood - the world was a totally new environment. If all of the animals are young, then their most ferocious enemy is also going to be young. While the exact picture of what happened after the ark is unknown, if the skeptics are going to speculate, then those who are not so skeptical are also permitted to speculate.
The question remains however, does the bible command that only ADULT animals were permitted to go on the Ark? Or rather does it FORBID Noah taking infants onto the Ark? And the answer for both of these questions is NO. It is therefore quite plausible and reasonable to speculate that such animals were indeed infants or at the worst - not full grown animals. This of course exposes the skeptics as not doing their due diligence before attempting to refute the story. It reveals a bias.
Those who wish to reply to this OP - need only address the particular aspects of whether the Bible forbids infants from entering the Ark or rather the Bible only commanding that adults are put into the Ark.