Newbie question
 

My tank came with a TDS tester. It has two functions, hc and ho, but give almost the same result. What is this used for?
thx

The functions HO and HC are for HOLD and HOLD CANCEL. If you're taking a reading, the display will constantly sample and update. Press HO to freeze the current reading. Press HC to release it.

Simple, yes?

I should have been more generous in my reading. If your question was what to HC and HO do, I've answered. If you are asking what's a TDS for...

It measures the Total Dissolved Solids (impurities, effectively). It is typically used to measure the relative effectiveness of a water purification system. There are lots of posts about TDS here that a search will uncover.