Undergravel filter. Yes or no

[TheHouseOfIllRepute]

My cold water tank is 120l
I have a fluval filter but it also came with a filter, I forget the brand
I always use a gravel cleaner during water changes
We moved the tank and I was shocked how much crap was stirred up.
My snails died but fish and shrimp seem their usual lively selves
I've put both filters in but thought perhaps an under gravel filter would do a better job
I don't understand how they work
When I look to buy they just look like a plastic tray and it's not clear how they pump or filter

They don't pump or filter. They just provide a good surface for the good bacteria to colonise as far as I remember.

If you clean your filter and your gravel all at the same time then you are losing a lot of the bacteria that is needed to digest the fish waste (ammonia).

But when I clean the sponges they are really dirty
I clean them in water from the tank
The water is very clear

Do you test your water for the levels of ammonia/nitrite/nitrate? That's more important than whether your filter sponges look dirty. They are supposed to be full of good bacteria. As long as the water can flow through and they aren't clogged then it's not a bad thing.

Having to clean and change your filter sponges so much is a myth - if they're falling apart they need changing, if they aren't then they don't.

Clear water is good, but fish can ail in water that looks crystal clear but that is high in the wrong levels.

I have only taken my water to a shop for the chap to test for me
He's ok but doesn't always give good advice. He told me I could have far too many fish in my old tank
I need a testing kit then
Are they easy to understand and can anyone recommend any particular one