You dont have to confront or write an email. Talk to him in a casual way and please be nice, but you have to be strong hearted becoz your father may say these things:

1) 6' aquarium is not cheap. I don't have the money.
2) We just bought a 3' tank. I don't want to spend for a 6'
3) We don't have the space.
4) Don't waste my money. We need money for other things.
But u don't have to worry. This is a norm. Most fathers will behave in such a way. U just keep quiet, smile and say nothing. Unless:

1) He is asking the reasons u need it; then u explain.
2) His reason is it is not good for u; for e.g. it will deviate from your studies; then in this case u have to convince him.
Other than the 2 situations u just look him directly in his eyes, smile and say nothing. Don't argue. Becoz u won't win the argument. Your target here is to persuade him and not to force him. U will achieve nothing if u argue or quarrel with him. Remember, it is his money!
Wait for another 3 or 4 days, then ask again those questions that I have suggested:

1) Dad, I love to have a 6' tank.
2) Dad, is it ok if u buy me a 6' tank?
3) Dad, my friend's aquarium is nice. Can I have it, please?
4) Dad, if I really study hard this year and get to be top 3 student, can I have a 6' aquarium, please?
Repeat this cycle again and again. It may take some time but if u be patient and continue with this method u will get it. But I may caution u that for a stingy father it may take longer, may even extent to 1 year. But average about 6 months.

He may then slowly make a research on the price of 6 feet aquarium without your knowledge, finding a cheaper manufacture or seller and then he'll buy for u.

U may discuss with Devillsin and improve this method as he has successfully persuaded his father. Of course his father is different from yours but most fathers will do as what I said above.

Congratulation Ben for your new 6' aquarium!

Bensow, get your 6 footer 1st. Then, start to think about other things. 
You need to learn how to focus my young padawan.

Ben, please re-read my suggestion. You should not answer any of the 4 questions, because that will lead u to an argument with your father. You won't win the argument.

When u ask him to buy a new tank, u know that the answer will be 'no', so what u do is just paste/register in his mind that u want a 6' aquarium and by keep repeating it every 3/4 day, u want him to remember it. U want to make him thinking about it.

U should also try to play with his emotion. This is how u do it:

U love keeping the arowana/other fish so try to share your happiness/hobby with your father.

1) Let say u're feeding the fish, do it when your father is around and show him. "Father, look at the arowana. The way he fetches the worm is very interesting. He is very aggressive in snatching it!"

2) When u read some interesting facts about the fish, tell it to him. Explain those facts to him.

3) Your weekly routine of changing part of the water, do it while he is around so he may see your high interest in it.

When u do a research on the price of the new aquarium or any good offer of a second-hand aquarium, always tell your father. Print the picture of the aquarium and show it to him.

But of course, all these must be done in a casual manner, the nice conversation between father and son.

Believe me young man, if u do as I suggest, u will get the 6' aquarium of your dream!