I'm a big fan of DSLR and mirrorless video, and the looks of the large sensors (small depth of field). However, for your goal, small depth might be a little difficult because when you get up close with not too much light (esp. in the lower corners of your tank) you need to shoot wide open and therefore the focus area might be too shallow. On the other hand the larger sensors are better regarding noise. It's a tough decision.
If you have friends who own a DSLR and a videocamera, ask whether you can borrow it and check the results in your aquarium store before you buy.
Regarding the slider: shooting aquarium glass from the side will give you blurry images (if I remember right, only when there is water in it). 90° is your preferred angle. Using the slider, you would have to rotate the head to keep objects that are so close within the frame, and that would result in shooting at an angle to the glass that is not going to produce good results. What I want to say is: you probably don't need a slider. Wow, did I just say that? I totally love sliders.
The 29 minute recording limit shouldn't be an issue for your kind of work, and the zoom shouldn't either. Hardly any Hollywood movie, documentation, sitcom, etc. ever uses zooming while recording.
What's much more of an issue is focus while recording. Your fish and critters move, and your focus should work really well unless you want to get familiar with follow focus devices. The a6300 does an amazing job, so do most videocameras. With DSLRs there are difficulties at times.
Last but not least codec. Sonys XAVC-S codec is great compared to what most DSLRs offer.
And one more: if you think about talking in front of your camera, consider an external mic. You can record external, and synch in post, or have it easier and record into the cam. A lavalier mic on your shirt, or a shotgun mic above your head just out of the frame are good options. Rode has a nice lavallier for the iPhone in case you have one and that is probably the cheapest route.
