My last 3 closings involved septic tanks. One failed, the others passed. One was new. (new tanks can still fail) One required a complete new system. One had a 2-day flush test. The others did not. One had lots of trees and therefore lots of 'roots" growing in the fields and around the septic tank and D tank.
Every situation is different. The companies that I recommend are different depending upon the circumstances and the location.
Feel free to provide more details or contact me directly.
Would be happy to help......but need additional info.