A properly supported and constructed acrylic tank should not crack.  Acrylic has more flexibility than glass, however, it is still relatively brittle and can crack under rapid temperature changes and mechanical stress.

If your acrylic tank does crack, then the first thing to do (after making it safe, for both yourself and the livestock) is contact the supplier and / or manufacturer.  They will be able to provide advise directly as they will be fully aware of how it was constructed, dimensions, and the appropriate repair technique.  It may require the tank to be returned to the manufacturer for repair.

Whether it can be repaired DIY satifactorily depends on how severe the crack is and your own personal experience with working with acrylic.  If you haven't don't so before, then for something as major as  tank it would be preferable that you consult with someone that has worked with it previously, or have them perform the entire repair.

Using silicone to "repair" a break in acrylic will not work, since silicone will not bond very well to the acrylic.  It can work in non-vital locations, such as for an internal weir.  But it should not be relied apon for something as pivotal as the tank itself.

Very small cracks that are not very long or don't go the entire way through can be repaired using heat.  Gentle heating of the acrylic (and the emphasis is on gentle here, heating too quickly will cause the acrylic to blister) with cause many fine cracks to then close up entirely.

Slightly larger cracks that are wide enough for a solvent to penetrate can be repaired using something like Weldon 3 or less.  Such a runny glue as this can penetrate into the crack and will seal it.  Flexing of the acrylic panel to slightly open the crack, filling with glue, leaving for about 30 seconds then closing, will assist in penetration and sealing of the crack.

More major cracks all the way through the panel or along the length/width can be repaired in the same manner.  If not confident in your gluing technique, then an additional sheet of acrylic overhaning the edges of the crack will add more strength.

For a major crack that stops away from the edge of the panel, to stop that crack from possibly propagating further either repair it using the heating technique, or drill out the end of the crack.  Much the same technique that is used for glass.  This reduces the concentration of stress at the end of the crack, and decreases the chances of it being able to propagate further.  Then refill the hole drilled using something like Weldon 40, which is a two part glue that actually forms acrylic in addition to bonding to existing arcylic present.  It works very well for filling in holes or cracks in this situations.