I would invest in a sump, personally I love it and will never go back to canister filter lol IMO. First I would defiantly sort out your fish, although they have been living together, it's not for there best interest especially for the gold fish. Im unsure whats your future plans, but if you can I would just jump right into getting a 180 or bigger to keep a retic for life or at least majority of it's life. With this you will have more water volume, especially if you decide on a sump which will add in an extra 30-50 gallons of water.