You may have to figure out the type of septic system you have first since there are a few different designs. You may be able to contact your local health department to see if they have a record of when your system was installed and the type it is though sometimes their records are incomplete if the system is more than 20 years old.
Usually you don't want to overload your system by adding too much waste water in one day so if you have a lot of aquariums or large ones, you can stagger water changes during the week.
You also should find out where your sump pit water is draining to, usually it doesn't drain to the septic tank since they are usually installed to drain footer water around the perimeter of the house and that water doesn't 'need' to be treated biologically.
Hopefully your yard work people have some knowledge of working in the 'septic' zone section of the property so they don't cause any future problems.
Having said all the above, we also have a 'septic' system though ours is a Jet Aeration system and doesn't have the multiple leach drain type of system. It has a single vapor ditch that is used to leach the minor effluent at the end of the biological process. All our household water goes into the system except the water softener discharge which is piped separately onto the property. Our fish room drains 300G per week into the 'septic' system with no over overloading issues and our other tanks drain 300G per week directly to the back yard.
Hope this helps!