You don't have to place a bucket of water over the tank. It doesn't look like there's any water in it, but there is. If you want a blue tint you can put something blue in the background.
I remembered there was discussion of this back when fishing was in beta testing.
So after some digging I found the thread from 4 years ago, and I even moved it out where you can see it,
>here<The gist is this:
Jack wrote:We already thought about extendable fish tanks and I even coded the behavior but there are side effects which are impossible to fix which makes it almost impossible to have unfortunately.
*10 hours later*
Jack wrote:I managed to make them work horizontally with some dirty hacks (like warping home a fish which exited its own tank when the tank containing it is destroyed), but I'm unsure about making them stackable since stackable support is handled in a custom way for specific things (see addons, chest and similars)
1. Junk-Jack-Wiki2. The world is big, 256 blocks high and 768 across.
3. Who is John Galt?
4. Lack of objection does not indicate consent.
5. We scream, we shout, we wail, we cry; but in the end we all must die.