Seems to me you have a legitimate claim on the contractor's bond - too coincidental for those to show up in what appears to be a fairly well established tank. If they were doing work on the sewer, I'd contact the owner of the project (the City or Water District?) and fill them on the problem. Perhaps going through them instead of the contractor may help to get some action rather than blanket denial.
Have you contacted a LFS
that also does tank work/commercial installations and maintenance, and see what their opinion is? Seems like that would be a good "unbiased" opinion to present to the contractor and project owner. Saltwater City in Bellevue and Barrier Reef Aquariums in Renton come to mind.