What do the plus and minus signs generally mean on a sponge filter?

Question

It is just how powerfully the filter is working?

Also, is there ever a danger of a fish swimming into the filter's hole if it is small enough?

Answer 1

is it on a k**b on the outside of the filter? then yes, its just the power settings

if its on the sponge its telling you which way to put it in.

fish can swim into the hole if it is big enough for them. the only fish that this is a concern for is fry.

Answer 2

I think it tells you which way to put the sponge in.

I have small fish and I haven't seen that yet--I have an average filter, so I'd think that your fish would be okay.

Answer 3

Yup it's for water flow. Very tiny fish can get sucked up there. Not just fry. I found a small white cloud fish in my filter when I was changing the sponge. And those little aquatic frogs can get their legs sucked into the filter and get stuck pretty easily. You can put a net around it if you are worried about it.