Hi again

we have two oscars one is roughly approx 20cm and the other between 10 -15 cm

would it be safe to put them together?

as long as your tank is large enough(about 100 gallons or so) to support 2 oscars then you can try and see how it goes. just remember tohave a back up plan in case they do not get along.

is about 120 liters

your tank works out to be about 30 us gallons and unfortunetly that is no where large enough for one oscar let alone two.

we recommend a MINIMUM tank size of 55 gallons(appro. 200 litres) for one oscar and 100 gallons(appr. 400 liters) for 2.

oscars can and will grow quite large(over 12 inches in length) and they grow very quickly. the main reason large tanks are required is because that oscars are extremly messy fish who produce large bio loads(waste). if you keep an oscar in a tank that is too small you will run into water quality problem which in turn will cause sickness in your oscars. also the maintenance that is required in a small tank would be overwhelming in a short period of time.

imho you should look at upgrading your tank and filtration as soon as possible to something more suitable for your oscars.