There are several different ways to look at this. A lot depends on your top priority. Is it budget? Is it performance, regardless of cost? Is it ease of use? Is it reliability? How you answer those questions will go a long way in determining which is the best course for you to take. We'll assume you're wanting to spend as little as possible without compromising usability for your specific use case with shooting macro images of a fish in an aquarium.
First let's look at what you don't need:
- You don't really need TTL or any other automatic flash modes if you'll be shooting under pretty much identical ambient light conditions all of the time.
- You don't really need to be able to control the flash power from
the camera when your camera is going to be within a few feet of the off-camera flash. It should be fairly easy to adjust the output power of the flash directly on the flash's controls.
- You don't really need a flash with a zoom head that widens or narrows the beam for lenses of different focal lengths. Shooting down into an aquarium means pretty much all of the light from the flash is going to bounce around all over the place. You might even want to use some sort of diffuser on the flash to soften the light up a little if you don't want specular highlights reflecting off the fish.
Once you get your setup dialed in you'll likely shoot the vast majority of your photos using the same settings so the drawbacks of manual only flash won't really apply in your situation.
Now let's look at what you do need:
- Controllable Light. That's about it. Pretty much any flash will provide it.
Obviously the more consistent it is from shot-to-shot the less you will need to post process each shot individually if you even need to do any post processing at all. If you decide to later add additional flashes you would only need a manual flash with a "dumb slave" mode that could be triggered optically by the firing of your main overhead flash. Using the same model as your overhead flash should allow consistency in the temperature of the light from each additional unit.
If your camera has a pc sync^ connection (the D3200 does not, but many other cameras do), then the simplest and probably cheapest way to go is to run a short pc cord from the camera to a basic manual flash with a pc sync connector. If the camera doesn't have a pc sync connector then an off-shoe flash cord running from your camera's hot shoe to a basic manual flash would work the same way.
(^PC in the context of flash photography has nothing to do with a personal computer. It is an abbreviation of Prontor/Compur. Prontor has its origins in the Italian word pronto (quick) and was a brand of shutter produced by Alfred Gauthier in the 1950s. Compur, derived from the word compound, was the shutter brand of the Deckel Company. Both companies were based in Germany and both counted Zeiss as an influential stockholder when they introduced the standard 1/8"-inch coaxial connector for shutter/flash synchronization.)
SB-26 Unless you are planning to run a sync cord/hot shoe cord from your camera to the flash, the SB-26 is probably not a good fit for what you are trying to do. Due to the glass between your camera and the fish you don't want to use an on camera flash to trigger the off camera flash mounted above the tank. Without an additional transmitter/receiver set or sync cord the SB-26 can't be fired remotely unless there is another flash, presumably on the camera, to trigger it optically.
If you do go the hot shoe cord route, then the SB-26 is a better solution than it would be for a wireless scenario. Name brand flashes are generally more reliable than third party flashes. It has a guide number of 50 meters (ISO 100, 85mm lens on FF camera). That should be more than enough power for the use you are considering. The SB-26 is an older d-TTL flash and won't be usable as an i-TTL with your D3200, but will be usable in manual mode and with the flash's Auto-thyristor mode. Of course in the case of the SB-26 you're also buying used and need to take that into consideration. If you do have any problems you won't have any kind of warranty from day one.
YN560 IV It would be better suited in terms of wireless triggering without a camera mounted flash, but still not the best fit for what you want to do. Although it can be triggered wirelessly by using a YN560-TX transmitter mounted on your camera, you'll be paying for the built-in transmitter in the flash that you won't need. The YN560 III does everything the IV does except for the built in transmitter and can sometimes be found a little cheaper. Both would also work with a pc sync or off shoe cord which would eliminate the need to buy a YN560TX transmitter. Both have a guide number of 58 meters (ISO 100, 105mm lens on FF camera).
Other flashes There are other manual only flashes available for a little less than something like the YN560 III/IV, but you give up a bit in terms of power and features. I wouldn't recommend anything that doesn't allow you to control power in 1/3 stop or, at the very least, 1/2 stop increments. This roundup from Flash Havoc a couple of years ago lists what they consider the biggest bang for the buck in off camera manual flashes.
Adding additional flashes If or when you reach the point where you may want to use additional flashes the simplest way to do this would be by using flashes capable of functioning as an optically triggered slave. An optical slave is triggered by the light released when the master flash fires. The master in your scenario would be the one wired to your camera that is mounted over your fish tank.
For some uses an optical slave with a rotatable head is desirable since the optical receiver works best when pointed towards the master flash. You would then rotate the head of the flash to point to your subject. But in your scenario there's going to be plenty enough light bouncing around in the aquarium to trigger any optical slave pointed towards the tank. Both the SB-26 and the YN560 III/IV have optical slave capability. So do most of the other flashes listed in the Flash Havoc link.
When using a manual flash triggered as an optical slave, all changes to settings must be made using the controls on the flash. This can sometimes be a pain if you need to change settings frequently. But with shooting an aquarium you'll probably dial in a particular setup and stay with it, so you can save money by not paying for the ability to control the flash power and zoom (if the flash has a zoom setting) remotely. And it is not really that far from one end to the other of most aquariums.
A word about reliability. Third party flashes have reputations of being less reliable than brand name ones. In general though, you can buy 3-5 third party flashes for the same price as a new name brand flash with comparable features. I use a mix of both. I carry one Canon flash in my bag and use multiple Yongnuo flashes. If a Yongnuo unit fails, it's disposable. (I've had one Yongnuo YN568EX II die after 14 months without any externally inflicted damage preceding the failure. It started showing signs it had a problem about two months earlier, just a week or so after it was out of warranty!) If shooting in situations where a do-over later is impossible, I always be sure to have one more Yongnuo with me than I need (or the Canon as an unused backup). And when shooting in crowds (that usually have been drinking alcohol) I'm much less worried about a guest knocking over a stand and destroying a Yongnuo flash than I would be if there were a $500 Canon flash mounted on that stand that just got clobbered.