Ok .... I know I may get a fishy flaming but I do need genuine opinion.
Four years ago we decided to get fish for ds's and our own pleasure. I had liked the idea of tetras (very warm house) or minnows. Went to pet store, explained had tank set up - they suggested we get fancy goldfish. I said no as too big and messy, the sales lady pointed us in their direction and ds fell in love with them, they were weeny. The sales lady then told us that the ones she had shown us were bred not to get bigger.
We left with two.
We now know that she lied 
Last summer I decided we need to get a larger tank - this time I decided to go to a well known pet superstore for advice. Long story short they sold us a Biorb! I have since - thanks to Mumsnet realised this wasn't good advice. I now have one goldfish left in a 30l Biorb (the other died 6 weeks ago), I maintain and check the water regularly. I cannot afford another tank and don't have the space for anything larger
.
I got advice from World of Water who said my options were rehome the fish to someone with a large enough tank as iot will just keep getting bigger - I don't know anyone.
Or
Maintain his lonely existence (I daren't put anything else in with him)- in the biorb.
Can I do this, keep him in the Biorb?
Am I horrible to do so?
And how the hell do you rehome a goldfish anyway? 
Sorry so long.