Sounds like typical "New Tank Syndrome". I would recommend purchasing an Ammonia/Nitrite/Nitrate test kit and monitor the Nitrogen Cycle in your tank. Don't add new fish yet but do a water change from the bottom to remove excess wasteproducts. Let the tank run a few days. Buy one, cheap fish, like a feeder guppy, and wait until the tank is cycled, before adding more fish. What size tank is it?

You really need to research a lot. Research what is cycling, how to maintain your water, the PH and temperatures + water requirements of different fish, etc. Water change frequency is important too. I notice that your shark "jumped out". You may want to purchase a lid with some good light, a thermometer...
Easy fish that can take a lot of abuse are convict cichlids. They are like the pitbulls of the freshwater ;)
They like a pH of 7 and a temp of 75. However, do not throw fragile fish with them because they can get very aggressive.

You could take a water sample to the shop where you bought the tank. By testing the ammo and nitrite levels they can tell where you are in the cycle. Plattys and mollys are good to cycle with [zebras are to]. If the store won't test your water go somewhere else. Eventually you will probably want to buy a test kit. Your ammo and nitrite will hit a peak then drop suddenly. Your water will never be that good again. I wouldn't do a major water change now as it may impede the cycle.

First of all, get test kits for ammonia, nitrIte, nitrAte and PH...avoid the strips as they are not very accurate and get the drops. I assume you have a thermometer, the temp should stay steady in your tank and not fluctuate, which can be a problem if it does because it means your heater is not working properly. Temps for most fish to be comfortable would be in the 76-78 range, a degree or two either way won't hurt them as long as the temps are steady. Anyway...if the params are good, then you can add a couple small fish (NOT mollies). If not, you can keep your cycle going, if it's still cycling, by adding small amounts of fish food to the water and don't do any water changes. This is a fishless cycle, as the tank doesn't need fish to cycle, just something to keep the ammonia and nitrItes going until the beneficial bacteria have had some time to colonize in your tank. Once the ammonia and nitrItes are at 0, your tank is cycled and ready for a couple fish. Don't add too many at once, cuz you'll have an ammonia spike that might get to high and kill them. And a thought on PH...most fish can adapt to whatever ph your water is. A steady ph is more important than trying to get the 'proper' ph for your fish. Therefore, don't mess with it, and don't believe those that tell you that you must have your ph at such and such, just brings more headaches. Only the more difficult (more like delicate) fish is it important (discus come to mind). My two cents..

Karen

You don't mention any thing, size of tank so on. Shark jumped (many fish try to escape from toxic (read poisoned) water thinking (instincts) they will be in another better puddle, alas!

Don't do anything until you do your research FIRST. That way your fish won't suffer. Always understock. If it is say a ten gallon tank, then a small school of this or that.

Start with your tank size.

Determine your local water ph and hardness (as other mention get those test kits and thermomentor et al).

Decide on type of fish based on tank size and fish requirements and your experience.

See what fish goes with what. Go with less, any way most experienced will not overstock as they know better, lol.

Live plants (always best) but hard for beginners so use easiest depending on light requirements. There are some easy beginner plants. If not go with silk, they are very pretty now. (although of course they don't help water quality but at least helpful to secure fish). See my other posts on easier cycling but if you start slow and with only one or two fish it will go much easier. What type of 'shark' (not true names and many get huge and freek in smaller tanks without hiding spots caves wood so on). Again, read up on types of fish and their needs. all have different needs. Guppies and mollies need some salt, some fish need softer water some harder.

Then half of your problem will be solved. If you are still reading this thread, post tank size and what type of fish you like. Then others can assist better.

Many beginners overstock, over feed, under clean, wrong fish size for tank (esp gold fish, bettas if bowls so on). There are great sites that mention fish and their needs. Just yahoo or google.

Best Sherry

hi all
my tank is nw 5/6 weeks old and is stocked up,i no you are supposed to cycle your tank which i did for the 1st 3 days,because according to a site i found on the computer after 3 days of cycling you could add your fish,which i did they where 8 glowlight tetra's and since then added more,i have had no problems with my tank and the fish are breeding and i also have new borns in there too.
i ether had extremely good luck or the fish are the hardy type,either where no problems with anything now,water crystal clear all fish feeding,i also have 2 large wood shrimp in there too wow so cute (lol) and was told if they survive then water quality good

yowz, i don't know what site you were reading, but cycling for 3 days without any bacterial additive is NOT sufficient. there are extremely perishable (expensive) "good" bacterial additives that you can throw into a tank to cycle it in a week, otherwise you have to wait 'til the load of aerobic bacteria builds up to sufficient levels. they are ubiquitous, but can be introduced via live plants, old filter material from an established healthy tank etc. cycling a tank can take up to a month for a large set-up. and yeah, test your water often at this stage.

and i disagree with adding feeder fish to cycle your tank. they often do more harm than good by introducing bacteria/parasites. a hardy non-feeder fish should suffice (fancy danios are good). even with a 60 gallon tank, i never add more than 6 fish at a time.

this is a little after the fact, but i thought i'd throw my 2 cents in.

-mint

Just to add. Been a while since I had a tank but for max. ammount of fish I would use the rule for every inch of fish you need 1 1/2 gal. of wateror so. ie: 10 gallon tank= 10 inches of fish or less. A 2 inch long goldfish=2 gallons of water so not to over crowd.

Wow cycleing in 3 days would be a record, don't take advice from the person that said that. It takes weeks. Just be careful at this point [don't overfeed] as the tank can crash quickly. Regular water changes and an test the ph and ammo every so often will help.