The 50mm is quite a good lens for taking pictures of your aquarium. This is where I would start. I guess it is the 50mm f1.8 which is much better at low light (and aquariums are usually low light) than the 70-300mm you are considering. What is it that you think you miss with this 50mm lens when it comes to photographing aquariums?
I just checked in my aquarium books. Now it gets a little complicated, so just skip the italic part of the text if you are not interested in the "tech" side of it:
Amano for example used 8x10 film for most of his famous images. He used 200mm or 300mm lenses. Since he used that big film instead of our rather small image sensors, you have to calculate a "crop factor". The diagonal of 8x10 film is 300mm, the diagonal of your 60D sensor is 26,82mm, so around 1/11th. So you have to devide the 200mm focal length of Amano by 11, resulting in around 18mm. And around 28mm for his 300mm lens.
That would mean to reproduce the look of his images, you´d need 18mm to around 28mm lenses for your Canon 60D. That again would mean that your 50mm lens is way better suited than the 70-300mm you are considering. So start with what you have and think about what you are really missing before buying new lenses. I do understand the wish for the longer focal length for wildlife photography though, but for the aquarium, you don´t need it, except for close ups of course.
Sorry für the techy stuff, I usually try to avoid that, but I wanted to make a point so that you know what the best of the best use to take pictures of aquariums.
In regard to the tank and your fish. Which fish is that? I don´t want to be a smart alec but just to inform you if you don´t already know: most fish (with very few exceptions) are social creatures and like to live among their peer

. Also many like to hide for sleeping, etc.. If they don´t get that, they usually don´t live up to their full beauty and eventually die.
Amanos work was awesome. He was the first to start these kind of tanks. Nowadays more and more people "copy" his work.
What lenses do you currently own?
I´m not sure what you mean with volcano-like photos, guess it is the colors of the plants in your tank?
Thanks

I do own a few sigma lenses. As always it depends on what lens you buy. Some newer ones are even better than their Canon counterparts, but I´m afraid I can´t tell you much about the one you are referring to. To me it feels a little long (focal length wise)
I see - when you are visiting oceanariums, you´d usually have some people standing in front of them and the rooms are often quite narrow. For this you´d usually want a wideangle (short focal length) to reduce the chance of people in your image.
When photographing tanks you also usually want to photograph in a 90° angle to the front glass (unless you have some bad reflections from this angle). Reason being is refraction of light. Your image will get blurry the more you shoot the glass from the side. Of course
for images like this you have to live with it.
BTW: this was one of mine:
2002 AGA Aquascaping Contest - Entry #58
Oh my, your photos are stunning, man. I wish one day I could make something like this.
And thank you for the tips! My other tank is full of malavi cichlids and it'll be great to practice in such photography.
Have you ever used Sigma lenses? I have several Canon and don't know about how good others are and asking it in the professional firum seemed like good idea ^^'
I thought about this one 'cause I'm an animalist mostly and I need telephoto lenses for visiting zoos but also I'm in need of lens for aquariums and macro... My friend told me that there're some good lenses from USSR that I can also use on my Canon but they don't have AF on them and I'm a beginner for full MF using.
My goal is to make some volcano-like photos with my fish in his black'n'red tank and for now I'm reading about Amano (thank you for letting me know about his works!).
I'm glad I visited this forum.
I own Canon 50mm as a lens for portraits on my canon 60D. I tryed canon 7D but is was worse than my ol' good 60D and so I think I'll continue using my oldie. Now I take photos only of my dog as the beginning (he likes to pose and it helps alot) and it gives me practice by taking him in still or active poses.
About volcano-like... I'm fond of volcanoes. And my new pet fish lives in a tank with black'n'red sand. The tank is empty (without decorations) as I will use is as a "photo-studio" for him. Now I'm searchin on
Ebay for something interesting I can put into the tank to imitate volcanos' lava and so on. I've read about black fabric to put on the tank for the better vision and also I have led lamps (searching for lava lamps for aquariums). But it's all impossible without macro lens xD