Even with a septic tank, the quantities you are using will be harmless even if you should slip up and pour the tank full of recently used developer (HC-110) down the drain.
And as for the fixer, you should be re-using it anyways. The tiny amount that will go into the waste water with your wash water won't hurt anything.
After the fixer has been used to its capacity, the fixer itself is relatively harmless, but the silver in it can cause problems with septic tanks. So careful disposal techniques are more important.
There is very little concern with the quantities used in home darkrooms.
If you find yourself doing more film, it would be wise to investigate silver recovery solutions - jnanian, who is a long time member here sells a "Silver Magnet" which is a good small-scale example.