Yea I plan on painting it with pond liner paint. But my question is will the trough hold after cutting out a larger window has this been done before?
Interesting question.
The thing you are asking is: if one cuts out enough of a side of the trough to allow for a glass window of x dimension, will the structural integrity of the trough be compromised? The answer, I would think, (not being a structural engineer) is that it depends on how it's done, what amount (size) is removed, and how the trough is originally constructed.
The chance that it will be fine I think is much greater if the window is smaller, rather than larger, simply because there is far less chance that a smaller window will affect the trough's overall structure. If you're looking at a larger window, I'd lean towards getting a purpose built trough with a preset window.
The price is tempting, but the cost of 300 gallons on a wood or carpeted floor could be prohibitive.
Assuming it's stainless or properly sealed, the trough still might have great uses as a sump, as a hospital or quarantine tank, etc. Just might not be the best candidate for a conversion.