We recently bought 2 simple goldfish, both about 6 cm long. We're trying  to give them the best care possible so they live a long and healthy life. We don't have too much cash to burn so we went with a tank of 40x27x24 cm. This holds about 26 liters of water. Will this be okay if we change the water once a week, or do we need an upgrade for them to be happy?