6 dead goldfish in 2 months. Is there something wrong with the tank?

2 Months ago I bought both of my children an aquarium and fish. Both tanks were set up in the correct manner, and both have been mostly looked after by me in the same way. However my daughters goldfish keep on dying, she has lost 6 in two months? Her tank is slightly larger than my sons but they have the same... show more 2 Months ago I bought both of my children an aquarium and fish. Both tanks were set up in the correct manner, and both have been mostly looked after by me in the same way. However my daughters goldfish keep on dying, she has lost 6 in two months? Her tank is slightly larger than my sons but they have the same filter system. The filters are designed for tanks up to 20 litres, hers has a 17.5 litre capacity.
Each time the fish have died the tank has been thouroughly cleaned and left to re-stabalise. Can anyone tell me what might be the problem? Do i need to get a bigger filter for her tank, her water does appear to get dirty quicker than the smaller tank. Please help, she is only 6 and its causing her great upset.
Update: PS didn't have 6 fish in the tank at the same time. just had to replace them as they have died. Also the tank was left for over a week to start cycling before any fish were added, and weekly water changes took place. Also does anyone know if I need to do a complete water change and do the cycling process... show more PS didn't have 6 fish in the tank at the same time. just had to replace them as they have died. Also the tank was left for over a week to start cycling before any fish were added, and weekly water changes took place.
Also does anyone know if I need to do a complete water change and do the cycling process over again, her last fish just died and the tank is now empty, or should I just take a water sample to the pet store to be tested?
Update 2: Water was treated with chemicals to remove chlorine and amonia, then left to cycle for a week.
Most of the fish died within 1-2 weeks of getting them.
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