Fix a crack in fiberglass tank with fiberfix?

Probably not. The reason is that you have to wrap the Fiberfix tape completely around the entire tank at least once, not just where it is cracked. If you don’t wrap it completely around, you will be relying entirely on the strength of the adhesive, instead of the strength of the tape itself, which is much stronger. 

If it were me, I would look at fiberglass repair kits that are specifically designed for this issue. They will almost certainly adhere much more strongly than any tape would. Note that thorough surface prep, i.e., cleaning and sanding, are critical. 

When you say ”fiberglass” you are only describing half the system.  Typically you have a cloth made of glass fiber which is impregnated with a resin that hardens.  The two most common resins are either polyester or epoxy, with polyester being most often used in large production environments like tank building or commercial boat building due to its versatility and good economics.  Epoxy resin is popular with small shops and hobbyists, but it costs more, is harder to work with but it does perform better.  The conventional wisdom is that ”epoxy resin sticks to polyester but polyester will not stick to epoxy” but I personally think that is a myth…with the vital determining factor being surface prep.  In other words you can get good adhesion only if you thoroughly scrub and sand clean the joining surface.  Lots of companies sell polyester or epoxy resin, and many are elitist and superstitious depending on the hobby they cater to (boatbuilding, aircraft, etc.)  My favorite no-nonsense supplier is Douglas and Sturgess (Google them), and their ”pdf” print catalog contains a treasure trove of expert advice.  Water is heavy, and fixing a crack in a 1,000 gallon tank could easily be subject to some very high stresses, so the structural character of the fix needs to be considered (and perhaps engineered).  The strength of the fiberglass needs to be sufficient to withstand the stresses, and the bonding area of the resin needs to be sufficient to get the adequate strength of adhesion.  Cannot advise those things remotely.

see:  https://www.youtube.com/watch?v=SNAztPE0HoA

The product to use is FiberFix ”Patch”.  That said you have to be quick. Note in the video they don’t waste any time.  If you don’t have everything ready to do the job just as FAST you end up with a stiff Patch that won’t bend or adhere. Sorry, buy another and start over.  This is somewhat true of Fiberfix tape; but, 10x more with Fiberfix Patch