I used those magnetic cleaners however some small pieces of eco-complete was caught and it ended up scratching my aquarium

They say that if your "finger nail catches" its too deep to repair.

I don't know what they means by " catches" Like if you can feel there is a scratch with your finger nail or that if your fingernail catches/get suck in the scratch.

My finger nail does not go inside the scratch but I can still feel that its there.

Thanks.

If you can feel that there is a scratch there with your finger nail, it means it is getting caught in the groove.

From what you have described, the scratch will be too difficult to repair without extensive rework.

I found that danger quite early with my first use of it. The inner piece dropped off as I tried to move the cleaner, and lay at the bottom with its face down. My substrate is composed of a mix of sand and laterite nodules. When I fished out the inner, its face was covered with hundreds of those brown nodules. These nodules are so rich in iron that they were attracted by magnet in the inner, and it was quite a task getting all of them off.

That started all the red flashing lights and the cleaner found its way in the darkest cabinet. I thought it a particular problem for my strange substrate - seems you all also should take a note of any grains sticking to the inner's surface.

I don't use magnetic cleaners for exactly that reason....I bought a "paint scraper" from amazon. comes in a variety of lengths and you can change out the razer blade....

I use this one but I am careful to only use the side with the blade and never let the fabric portion touch the substrate so it never gets any particles on it. The blade is fantastic and it makes cleaning the glass a snap. Mag-Flip 100 Aquarium Cleaner | That Fish Place