Hi! Welcome to the forum and to this wonderful hobby
You've been given some great advice here and been pointed in the right direction so I thought I'd chime in and add some extra detail.
First of all - cycling.
When fish eat they produce waste. The waste contains ammonia. Rotting food also contains ammonia which is why you must be careful to not over-feed. Unfortunately ammonia is deadly to your fish and causes permanent damage to their health. Luckily, after about a week, good bacteria start growing in your filter (which you must get ASAP - and I assume you already have a heater as your fish are tropical) and gravel/sand substrate. These beneficial bacteria convert the ammonia to nitrIte. NitrIte is still highly toxic. After a couple more weeks, ammonia drops to undetectable levels but nitrIte spikes and is at its deadliest. Luckily more good bacteria grow that quickly convert the nitrIte to the much less harmful nitrAte. You then do a weekly 25% water change to keep nitrAtes low enough. The whole cycling process takes about 6 weeks if done with fish and you should do a water change every 3-4 days now as your fish will get very stressed and poisoned otherwise.
The next thing to understand is that you MUST test your tank's water for ammonia, nitrIte and nitrAte regularly - especialy whilst in the process of cycling. Get a liquid-based test kit from your local fish store. Avoid the cheaper paper test strips as they aren't accurate and don't work after a certain period of time. The liquid ones are much better value for money in comparison. What you'll want to do is monitor the ammonia and nitrIte levels. Once they drop to 0, your tank is fully cycled and you can add a FEW more fish. You have to build up the number of fish gradualy as more fish mean more waste which means more ammonia. You need to build this ammonia up slowly so that the bacteria have a chance to multiply to keep up with it and to avoid having to go through the 6-week cycling process all over again and risk your fish' lives. You should also test for nitrAtes and aim to keep them below 40ppm though below 20 is better. A weekly 25% water change should be enough to achieve this provided you are not over-stocked.
When you do a water change you MUST first de-chlorinate the water. You see ordinary tap water is fine to use but it contains chlorine. The chlorine not only harms your fish but also kills off the good bacteria in your filter/gravel. This can cause a re-cycle and, as a result, fish deaths.
Another important part of maintainance is vacuuming your gravel (or sand) and occasionaly cleaning out the filter. Buy a gravel vacuum from your fish store and use it to suck up debris from the gravel and to help you do water changes. As for cleaning out the filter, never replace filter media (the sponges and filter floss inside) regardless of what the manufacturers instructions say. This is because all your good bacteria grow there and taking it out causes your tank to re-cycle. To clean out a clogged up filter, simply do a water change and use the old tank water to rinse the media out. Never use tap water as the chlorine in it will kill all the good bacteria in the filter. You only realy need to clean it out about once a month or when it gets clogged.
I should say at this point that a filter is incredibly important and you should get one immediately if you haven't already got one. Besides housing the invaluable bacteria and filtering out little bits and pieces from the water, it also aids water circulation and causes essential surface aggitation which allows oxygen in so your fish don't suffocate.
As for choosing sand - just don't get anything like crushed coral as this will alter your water's chemical composition (pH and hardness to be exact) and can stress your fish severely. Pool filter sand is cheap and safe. Make sure to rinse the sand thoroughly before putting it in or it'll cloud the tank and can take days to settle. Having said that, it's best to get through the cycle and have the tank running for a few months before undergoing the change as, right now, the tank is unstable and needs time to cycle and stabilise and the good bacteria need a chance to grow.
Hiding places can be anything that's made for aquariums. For the time being, avoid rocks or wood as, like crushed coral, these can alter your water's pH and hardness. If you do some research on rocks, however, you may be able to find some that won't cause such problems and buy only those. Artificial rock, plants and ornaments found at your LFS are ok to add now.
I've already been through the maintainance thing pretty much. The only thing to add, besides the weekly water change/gravel vacuum and monthly filter rinse, is that you may need to do some algae scraping occasionaly. Buy an algae scraper at your LFS and that'll do the trick.
Now about your current fish. Unfortunately they are all unsuitable for your tank. The colombian sharks, as has already been said, are a brackish species. This means they need sea salt (not ordinary salt) in their water. They also grow very big. Do some research on them - maybe a google search - to read some more on them. Also, though less urgent, the 'sucker fish' are probably common plecos. These are tropical freshwater fish but will also eventualy grow too large for the tank and are territorial. They'll be ok for now though as they grow slowly. They will not tolerate any salt so are incompatible with the 'sharks'. To be honest with you, the sharks should be returned to the fish store or re-homed.
Also, I think I said this earlier but it doesn't hurt to say it twice, don't get any more fish until after the tank is thoroughly cycled. Then, when you do, research what you're interested in carefuly first to make sure it'll work in your tank.
Oh, about speeding up the cycle - get your LFS to give you some old filter media right from one of their stock tanks. Choose the most healthy tank you can though so as not to introduce diseases. Put this mature media into your filter (cut it up if necessary). If they won't give you filter media, get some old gravel from one of the tanks and put it in yours. This media/gravel will contain a lot of the good bacteria your tank needs to cycle and should speed up the process tremendously. This realy is essential if the plecos (plecostomus catfish - the 'sucker fish' you mentioned) and sharks are to survive.
In future, you should do a fishless cycle before adding fish but nevermind about that now.
Anyway, very sorry about the long lecture and I'm sure I've missed stuff out. There are some links in my signature you may find useful as well. Good luck with your tank and fish!
/badmanstropicalfish.com/profiles/profile63.html
