Cooooooool 120 gallons

. I'll definitely subscribe to your channel in case you create one!
Then I'd say you need some lighting for the non-tank shots. Well, and that I'd say depends on how much you really plan to cover.
You can get really good quality video with cheap 500w lights from the hardware store, if you are a little creative and don't want to spend too much money. It's all a question of light formers. But these too can be DIY, using gels and creating a frame of wood.
Buying expensive lighting equipment only pays off if you use it regularely, then you pay for convenience and ease of use. But light usually is light, that can be formed and color corrected to your needs (beside cheap LEDs and fluorescents). If you have the tank and the equipment in the frame at the same time, you need to color balance the light so that they both have the same color temperature. That's similar to white balance and can be done with gel filters. If not, you just white balance in camera for the kind of light you use.
I do own reptiles and I use UVB bulbs for them to bask on but apart from that I need lights to light the terrarium so I could use those lights to illuminate the equipment when I film it.
If your lights are not too purple (I think I've seen purple ones for reptiles) and bright enough, that should work. That sounds like a cool plan, looking forward to the results

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And what about lenses, should I get a macro and the lens that comes with the camera or should I just buy the body and the macro and another lens? Also, how do I choose how many mm long it is?
I have to slightly disagree with advanced photo and with the article. Using these techniques from the article in a public aquarium should work pretty well. While your 120 gallon tank is huge, public aquariums are massive sometimes, so they give you other options. Plus: at home you can control light and reflections and there is nothing wrong wearing a ninja outfit

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Getting so close to the tank that you use a lens hood for removing reflections won't work too well for video, because you can't track the fish. Even slight rotation to left or right will result in aweful fringing. Same with wideangle lenses. Getting close to the tank will blur the corners of your frame, while shooting from a distance keeps those angles smaller and with it refraction.
Lens choice is difficult and depends on how far away you can get from your tank - if you can't, a tele won't work because they typically have a minimum focus distance of 1m+. And if you can't get far away from your tank, you dont need a lens that pulls it closer.
The longer the lens, the better your tripod fluid head has to be, and the less likely handheld will work for tracking a fish.
So a macro is key to some of the shots from the video you posted. Sony just introduced a new 50mm macro. The downside usually is, that they don't focus quick. I don't know about the newer ones though.
In my opinion 50mm is the perfect focal length for full frame sensors. Unfortunately there is no wider macro option for crop sensots to give you the same field of view. But that too should work pretty well. So I'd start out with that in mind.
BTW: try it. With amazon you can usually do that. If you don't like it you can send it back.