Probably the DUMBEST question on here in ages -- could I turn an old septic tank into a fish pond?

OK, I know this is a weird question!

My house was hooked up to municipal sewers back in the mid to late 1980s -- I think it was probably about 1987 or 1988. (Still have a well for water, though, YEAH!). At the time, the septic tank in the back yard was supposed to be pumped out (which it was) and "remediated" -- in other words, filled in. It was pumped out, but the contractor that installed the sewer line didn't come back and do part B. For some reason, no one ever pursued it, and the municipality didn't ever come check. Over the years, it just got ignored. I know it's under there, because when we have drought, that is the first place the grass starts to look bad, and the rectangular outline of the tank becomes clearly apparent.

OK, fast forward to now. The ground in the center of this area is beginning to settle down and form a depression, so its apparent the roof of this tank is now beginning to collapse. Per my mother, this tank is not a prefab unit, but was made in-place of concrete blocks and a wooden roof. The unit is approximately 4 x 8 feet in size, but the depth is an unknown -- my mother thinks it may be about 5 feet deep, but she doesn't know, and I have no idea what code would have required when this was installed (1949).

I have NO way of getting heavy equipment back there without doing significant damage to landscaping, which is something I don't want to do. So, I would have to fill it in myself, either with bulk or bagged soil. That sounds like a LOT of work. And, I'm not sure how many yards it would take 4 x 8 is 32 square feet, if it were 5 feet deep, that would be 160 cubic feet, which would be 5.92 cubic yards of soil. Yikes!

I was wondering if it would be possible to excavate it, cut off the boards, fill it partially so it's not extremely deep, stick a vinyl or rubber pond liner down there, and convert it into a pond.

What do you think? Pros, cons, dumbest thing you've ever heard in your life?