To protect yourself you're going to need a copy of what the post cleanup soil sample test results at closing, a document that proves that they properly disposed of the contaminated oil and sand that was in the old tank, a document that delineates the proper disposal of the tank itself.  

These ratings are just my opinion and I have no idea which investments will be successful and which will tank.  A

After the tenants move in it should be their responsibility to fill the tank as needed and make sure it is at the same level when they moved in prior to moving out.

Leaving the tank as empty as possible is never good, as you can get sludge build up at the bottom that can be sucked into the heating system.  

+1 if you can add in gas from the efficiency perspective, there are wall mounted propane heaters and may companies would lease the propane tank as well. 

If you can then you can rule out the septic tank as the problem and save yourself $600.2.

You can try to pattern them ..look for the support and resistance hoping to catch an ascending curve but it’s iffy because Patterns change and just some Bad news can tank a stock in one day .you just never know if your at the bottom .

Fill the tank 100% charge the tenant for the 1/4 tank as it is the lower of the 2 amounts you think it was.

We swap out the old gas hotwater tank as well and no more need for a chimney, or liners.If you are a diyer, why not knock out the chimney yourself, buy seconds or discontinued brick, lay the chimney yourself or hire a handyman.... most of the chimneys we needed to rebuild are only the top sectionsfrom the attic through the roof, its rare that the entire chimney is bad.

The tank is only designed to hold solids and then the excess liquid is continually being cycled through and out of the tank as you add more water.