Why it matters
The tank is the cheap half of the system to maintain and the expensive half to ignore. If it is not pumped, solids build up, the filter and baffles stop doing their job, and sludge washes out into the leaching bed, which is the part that costs $15,000 to $25,000 to replace. A cracked, corroded, or collapsing tank also leaks sewage into the ground. Steel tanks are the real worry, since they rust from the inside and typically last only 15 to 20 years, which is why many places no longer allow new ones. Concrete tanks often run 40 years or more, and poly tanks 30-plus.
How to spot it
A healthy tank is buried and quiet, so mostly you are looking for clues. Ask when it was last pumped and for the receipts, because a tank that has not been opened in years is a red flag on its own. On a visit, note a soft or sunken lid, any sewage smell near the tank, gurgling or slow drains inside, and whether anyone actually knows where the tank and its lid are. The state of the baffles, the effluent filter, and the sludge level can only be judged with the lid off, which is what a proper inspection does.
What it costs
A routine pump-out in Ontario runs $300 to $650 depending on tank size, and you should budget for one every 3 to 5 years. Replacing a failed tank is $5,000 to $12,000 installed, with the tank itself running $1,500 to $6,500 depending on material and size, concrete at the low end and poly or fibreglass higher. Replacing an effluent filter or repairing a baffle is far cheaper, in the low hundreds, which is the whole point of catching problems early.
What to do
Get the pumping history before you buy, and treat a tank with no records as one that is due for both service and a full look. Never park, pave, or build over the tank or its access lids, and keep the lids findable. Have the tank opened and pumped as part of a dedicated septic inspection so someone can actually see the baffles, filter, and walls. Then budget a pump-out every few years as ordinary ownership cost, not an emergency.
Education and triage, not a home inspection. Casaroo flags that a home has a septic tank and prompts the pumping and age questions, a licensed septic professional opens and assesses the actual tank. We flag; we don't inspect.