Well keeping an eye on him won't actually work you have 2 problems:
1) Size - Goldfish grow to the size that allows them to remain comfortable in their environment e.g. a goldfish bowl will never end up with a goldfish growing until its nose and bottom are touching each side of the bowl. A stunted goldfish develops premature organ failure due to an inability to grow to full size and thus the organs cannot reach maximum efficiency
2) The higher temps cause their metabolism to increase which cause them to grow more rapidly which provides poorer quality growth and thus premature organ failure normally of the kidney & liver if memory serves
A goldfish should live 20+years in the right environment.
A 30 gallon long would be perfect for that comet and it just needs filtering no heating bills required you could even skip the filtering if you plant it extensively and light it.