Some guesswork is involved here as you've managed to take pics and shoot a video that don't show one or two key details of the construction :-)
The obvious first thought is what you're worried about, that the bowing you're seeing is due to this being made from pine. Although pine is quite strong if used the right way the tank does represent a very significant load and it may be too much for the thickness you've used.
However I think there's more going on.
There are additional concerns in relation to how this is constructed, with the vertical supports cross-grain to the top. As I was writing this I realised there are actually two problems with this, not just the one I first saw.
The first is common to all furniture with similar tops, where there must be allowance for the panel to expand and contract with changes in internal moisture level. As we can't see how the top is attached (if at all) to the upright panels it's impossible to tell if this is an issue.
Switching to oak wouldn't necessarily be a solution as a result.
I was going to write initially that if you changed to top to plywood (see Note bottom on plywood type) it would avoid these issues — there are no worries with cross-grain attachments and it is inherently stiffer so will naturally try to resist bowing under load. But then I realised the following.
The second issue with the uprights is not so obvious but is potentially just as serious if not more so as it's still a factor even if the top were switched to plywood. Being built as they are their axis of expansion and contraction is vertical so when those are expanded they will push upwards on the top. This wouldn't be a problem necessarily, except for the steel posts to either side.
And conversely when these uprights contract they will tug on the top downwards if they're attached securely to it. If on the other hand the top just floats on them when they are at their driest point they could shrink enough to leave a gap between their upper edges and the top, which of course could be a direct cause of it bowing downwards.
Note: although it's a good grade of plywood it shouldn't be considered necessary to use marine-grade ply in this application, as although it is around water there isn't the expectation that it'll constantly get wet.