If I run the water in the tub for 3-5 seconds, that somehow triggers the toilet tank to fill up. Also, the water comes out of the sink super low flow... What might I be looking at with these symptoms? Thanks for your help!
You'd not mentioned whether every fixture in the house had low flow. Evaluation is step by step with many steps to uncover the low flow, depending on details. Can't solve via cryptic info.
is there high/normal flow elsewhere?
do you have galvanized pipes?
what is the water pressure?
you need a plumber or very well versed handy person