Why?

Have you ever played a mod that had annoying fluid byproducts that you had to get rid of? And you thought, "I could design a processing chain that will properly dispose of this thing... or I could put 20 tanks and then flush it manually once every hour."
If you want to stop yourself from cheesing this way and play Factorio the way it is meant to be played (automation!) then this mod is for you.

Goes well with Spilled Items.

Nullius

This mod is specifically made with Nullius in mind, where many processes create byproduct fluids that you have to work to get rid of. When using Nullius, this mod will only affect unvoidable fluids ("void in X" or "unvoidable"). Voidable fluids will still be deletable.

Mod settings

You can have different actions for flushing, removing a tank, or destroying a tank:

This might be useful if you really want to punish players from deleting fluids :D

Remote interface for other mod support.

If you want your mod to specifically mark some fluids as undeletable but not others, you can use the methods in the undeletable-fluids remote interface to mark specific fluids as undeletable.