what should i use for a cleaner tank and for my fishes good?

Just point the outlet of the filter up a little and it will keep the air plenty oxygenated.

Your main problem right now is that a 70l tank is way too small for 4 goldfish. Goldfish are really best kept in ponds where they have the space (and water volume) necessary for them to survive. I've kept a single goldfish in an 100l tank at home as the kids granddad bought him for them but after 3 years, I'm looking to re-home him with someone who has a goldfish pond.

Did you cycle the tank correctly? If you didn't, or don't understand the question, read the sticky at the top of this forum for a really good guide to doing it, bring the goldfish back to the shop where you bought them, follow the guide and then look at what kind of fish would be more suitable for a 70l tank. You may need to invest in a heater if you want to keep tropical fish but the good news is that tropicals are actually easier to keep than cold-water and you can get a wide variety of them that will happily co-exist in a "community" tank.

What shop did you go to btw? Was it a dedicated aquatics place or a general pet-shop? Because most of the latter are absolutely *USELESS* when it comes to fish. There's no need to feel bad for making the mistake of taking advice from a pet-shop, most of the regulars on this forum were in the same place when we started fishkeeping and I'm sure I'm not the only one who killed a few goldfish out of ignorance at the start!