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The rule for stocking tropical freshwater fish is 1 inch full grown per gallon. Meaning if the fsh you want gets 2 inches as an adult then you should have 2 gallons for that one fish. If you have a 10 gallon aquarium and all the fish you want to keep get 2 inches as an adult then you can fit a total of 5 fish in your aquarium. For saltwater we like to go off 1 inch full grown per 3 - 5 gallons.

The rule for stocking goldfish is 10 gallons per adult goldfish as even a comet (feeder/ fair) goldfish can get upwards of 8 inches in size as an adult. Depending on living conditions and diet, I've seen a 1 inch feeder goldfish get to 6 inches in length in less then 8 months.

Please note that things like live plants in a aquarium can increase the amount of fish you can have because these plants help keep the water quality more balanced for fish.

Don't be discouraged, this happens pretty often as some times the fish store employee might not have the time or knowledge on what is involved to ensuring success with your first aquarium. Unfortunately some stores have no problem selling you a brand new aquarium and a bunch of fish for it at once which can lead to new fishkeepers getting discouraged, the truth is that every aquarium/ aquatic ecosystem needs to go through something called the Nitrogen Cycle. This is basically the processs every container with water that is intended on housing fish needs to go through in order to safely keep fish alive. The nitrogen cycle usually takes 4 - 6 weeks to complete and during that time, beneifical bacteria aka nitrifying bacteria builds up. The job of this beneifical bacteria is to convert toxic ammonia into something safer for your fish, which is nitrites, then nitrates. This Nitrogen cycle will only begin once we add a source of ammonia to an aquarium which could be sprinkling fish food into a tank the first couple days of setting up an aquarium. Or some people will add one hardy fish to their tank and testing the water daily. Plus adding starter bacteria they sell at fish stores now either in liquid bottles or little gel balls is a good requirement for new fish keepers.

To summarize, if your tank is brand new, please be paitent and have an understanding of the nitrogen cycle before buying/adding fish to your brand new aquarium.

Generally you dont want the light on more then 6-8 hours a day. We should keep in mind that the fish tank recieves ambient lighting from the lights in the room and from the sun through the windows (it's not recommended to have your tank placed near a window as where the light hits the tank you will end up with algae on the glass). Also fish need their sleep/rest time just like humans so try to turn off your lights at night. I recommend a timer so it will turn off and on automatically everynight. Forgetting to turn off your light can lead to unslighty algae growth that needs to be manually cleaned.

Filters provide filtration to your aquarium helping keep it cleaner longer. Which means the water in the fish tank won't need to be changed as often. Filters also agitate the surface of the water which puts oxygen into the aqurium so your fish can breath. The reason you see fish like bettas living in cups is because betta and a select few other fish like gouramis have whats call a labyrinth organs which allows fish to breathe air from the surface of the water. These fish require access to the surface of the water, are sometimes known to jump and can build bubble nests.

As for heaters, check what the temperature requirements are for the type of fish you are keeping. Tropical fish do best between 75 and 80 degrees fahrenheit which unless you live in places light Southern Florida you will usually want a heater. Fish like goldfish or native fish would do fine at room temperature. Some cold water fish (goldfish are considered cold water) would require chillers to keep the tank at the proper temperature if you live somewhere where it's hot.

One should have a basic understanding of what the Nitrogen Cycle is. Water conditioners are required if your tap water has chlorine/ chloramines in it. Understand that not all fish get along with eachother. Understand that the 2 inch fish at the pet store might be 12 inches in less than a year so researching what fish you are getting is crucial. Beneficial bacteria is a must for a new tank if you are new to fishkeeping. Buy a water testing kit and test your water as often as you can when first setting up your first aquarium. Watch a video or ask someone to show you how to use a gravel vacuum/siphon. RESEARCH And most importantly, please don't buy a fish on the same day you buy your first aquarium as you are setting yourself up for a hard time. And if you are very in doubt if your fish are okay. Do a 25% water change with dechlorinated water that is close to the same temp.

Watch a video on how to properly use a gravel vacuum / siphon. There are different techniques to getting a siphon started so find which works best for you.

First get something to scrub the algae that is safe for either glass or acrylic depending on what your tank is made out of. Mr.Clean Magic Erasers are great for cleaning inside any fish tank as one eraser can be cut into 4 - 5 pieces and thrown away after each use. (note to only use the original magic erasers)

Use the siphon to clean the substrate/gravel of your fish tank ideally once every 2 weeks. While you use the siphon have the dirty water siphon into a bucket. If you have plants in your gravel it is recommended to simply hover over the plant to suck up any fish poop without burrying the siphon into the gravel. It's not require to remove your hardscape (large decor/stones/wood) everytime you clean an aquarium but its good to try to clean what can settle under large decor pieces at least once every 2-3 months. You should see browish colored poop water getting siphoned out of your fish tank that turns pretty clear (not perfectly clear). Your goal is to remove about 25% of the water while using the siphon to remove the poop that has settled into the substrate/gravel (more or less is okay depending on your system and needs at that time)

Some people will remove artifical decor each time they clean their aquarium to use a scrub brush to clean the algae off the decor in the sink.

Replace with water of a similar, ideally the same temp. Add your water conditioner within 10 minutes of adding water into your fish tank (or you can dechlorinate your water before putting it in your fish tank)

Replace the filter cartridge with activated carbon if its been over a month (some will rinse the cartridge if it looks really brown and dirty) Try not to rinise the cartridge in water with chloramines in it cause that can kill the beneifical bacteria on the cartridge. In filters with a double cartridge some will clean one one week then the other the following week.

Biological media should never be cleaned / rinised in anything but fish tank water as you do not want to risk killing your fish (dunk your bag of biological media a few times in the bucket of dirty fish tank water after you siphon)

Benefical bacteria lives in on the surface of everything inside your aquarium so if you clean everything but your gravel you will technically still have some good bacteria in the rocks of your fish tank which is what will ensure you don't accidently over clean your aquarium which can kill your fish.

Realize that fish come from lakes, rivers, the ocean which we don't clean, specially not with harsh chemicals, so the same applys to cleaning your fish tank. I've seen to many people try to use bleach and windex to clean their fish tank and they end up killing all the benefical bacteria thats keeping the fish alive or chemicals get into the water that posion the fish. Dechlorinated water, a scrub brush that hasn't been used for anything but fish tanks stuff, something to clean the glass, a siphon/gravel vac and a bucket are all you need. Some people will use a thermometer to try to get the temperature of the water as close to tank temp as possible before pouring the water into the tank. (if you are doing less then a 25% water change it's okay if the water isn't perfectly the same temp but if you do a big water change like 75% then get it as close to temp as possible)

Notice I did not say to remove the fish from your aquarium. In tanks over 5 gallons you really should leave the fish in the tank but it is normal to have a container / net to catch your fish out of your small aquarium to set the fish aside while simply using your hand to stir up the gravel then pour the water out or use a small siphon to change the water.

Pro tip: use a turkey baster for removing uneatten food and touching up smaller aquariums in between cleanings.