Hello makers!
This is my first instructable and I am going to admit that the photos provided aren't so many or of good quality, so I apologize. You see, I didn't intend to make an instructable of this build, but I then thought it would be a good idea to give some more information to those who might be researching this field...
I'm in the aquarium hobby for around four years now and the last year I've been constantly thinking about making an aquarium by myself. I've seen lot's of videos on YouTube and read lot's of things on forums etc, but still was afraid to do the big step and start glueing glasses together. You know, the thought that a significant amount of water could be running inside my house, seemed quite scary... So everything started when I had a conversation with a guy at the aquarium store I usually visit and he told me that he has a leaking 35lt tank laying arround. He said that he didn't have the time to fix it and he actually didn't care so much about it. So he was kind enough to give it to me, in order to experiment with aquarium building... Here we go then:

Things needed:
1) Silicone gun
2) Aquarium silicone or 100% silicone
3) Duct tape
4) Masking tape
5) 90 degree holding vices (at least I hope they are called so...)
6) Lot's of glass scraping and hobby-knife blades
7) Protective gloves (you can get seriously cut!)

Step 1: Disassemble the Tank and Remove the Silicone.

Without stressing the glass panels, use a blade to cut the silicone that holds them together. While manipulating them, make sure that you are wearing gloves or that you do gentle moves, so you won't get yourself cut. Also make sure that you don't hit the panels together, because they can crack fairly easily if the are thin like in my case (4mm). After that, you'll be left with 5 pieces of dirty and full of silicone glass. Now start removing the silicone from the old seams, with the help of a blade (you must remove ALL of it in order to get a safe seal later!). Notice that as you go on, the blade will become dull, so having some new ones nearby will speed things up.

This is the most difficult part of the whole built, because it's hard to remove old silicone and if you aren't careful, you can get yourself cut.

In case you need to continue your work another day, make sure that your panels are easily detected by others, so they won't break them accidentally and get injured. When I needed to do so, I simply stuck some pieces of red insulating tape on them and they were quite visible.

Step 2: Prepare the Glass Panels.

Use vinegar or a household cleaning chemical to remove the marks that salt, calcium and things like that leave on the glass surfaces. Wash them thorougly, taking care not to scratch them. After that, you are gonna need a place to let them dry. At than point, you should avoid doing the mistake I did myself, which was to stack them one on another... If you do that, then the thin layer of water that will get squeezed between the glass panels, will create a suction and then you won't be able to seperate them by pulling! The only solution if this occurs, is to slide them away from each other, so they will seperate. In order to avoid that, you can put some towels or sponges between the panels. Now let them dry, or use a hairdryer to speed things up and when done, apply some acetone to the surfaces where the new silicone will be set, in order to clean any remaining oils and stuff like that.

Step 3: Glue That Tank!

Ok, so now you think you are ready to go right? Well, you firstly need to make sure that some other things are taken care of.

Find the right place to work:
Silicone gets really smelly once curing has started, so you need to work in a well ventilated area. Keep also in mind that the aquarium must stay for at least a day where it was built, in order to gain enough structural strength to be moved without collapsing. So it seems that your garden might not be the perfect spot to work if there might be rain in the following days...

Ckeck your caulking gun:
Make sure that you have cut the silicone tube and the tip appropriately, so there won't be much effort required to get that thing flowing. Remember that you need to work fast and clean...

Dry fit your glass panels:
Figure out how they were glued before, so you know where to start from!

Use the masking tape:
Put some masking tape alongside the seams, so that you won't have silicone making a mess on your glasses.

Ask for someone to help you:
It's gonna be nice to have someone to hold those pieces in place for you. Just explain to them the whole procces beforehand.

Now you can start applying that silicone! Lay down the bottom piece and run a bead on where the first piece will sit. Put that piece in position, as vertically as you can and make sure to avoid moving it around too much because you could create bubbles inside the silicone, which will lead to weaker bonds and maybe even leaks. Continue untill all panels are in place and then use a spatula to get those seams prettier. Make sure all pieces are aligned and remove those masking tapes. Now it's time to use those vices to hold the panels in place and also tie the tank with some duct tape. And now starts the game of patience... Leave the tank to cure for as long as possible. I personally gave it one full week. You know... Better safe than sorry! I bet you don't wanna be "finding nemo" under your sofa...? Anyway, keep in mind that a smelly silicone indicates that the curing procces isn't over yet, so give it some more time if needed...

(I strongly recommend that you watch some videos about the glueing procedure, on YouTube, as it is a very critical step. The "King of DIY" has some awesome videos, so it would be helpful to check them out. Also, I wouldn't suggest to go for thanks bigger than 60 liters for the first time.)

Step 4: Water Testing!!!

Well, now that the tank is fully cured, take it outdoors (I took it to my balcony) and fill it with water. Let it there for another week and check daily for leaks, while adding any water than might have evaporated. If everything goes well and you don't detect any leak, repeat this procces for three days, indoors, where you can observe the tank more closely. After that, proceed with scaping it and starting the nitrogen cycle in order to bring this tank to life!

All in all, that's my short guide on aquarium building. It's not impossible to make your own tank, as long as you are doing things the right way. Get the appropriate tools, do some more research and go for it! Happy DIYing and greetings from Greece!