Hi and welcome to the forum
To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
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I recommend changing 75% of the water and gravel cleaning the substrate every week to dilute nutrients and disease organisms in the water.
Any new water should be free of chlorine/ chloramine before it is added to the tank.
You do water changes for 2 main reasons.
1) to reduce nutrients like ammonia, nitrite & nitrate.
2) to dilute disease organisms in the water.
Fish live in a soup of microscopic organisms including bacteria, fungus, viruses, protozoans, worms, flukes and various other things that make your skin crawl. Doing a big water change and gravel cleaning the substrate on a regular basis will dilute these organisms and reduce their numbers in the water, thus making it a safer and healthier environment for the fish.
If you do a 25% water change each week you leave behind 75% of the bad stuff in the water.
If you do a 50% water change each week you leave behind 50% of the bad stuff in the water.
If you do a 75% water change each week you leave behind 25% of the bad stuff in the water.
Fish live in their own waste. Their tank and filter is full of fish poop. The water they breath is filtered through fish poop. Cleaning filters, gravel and doing big regular water changes, removes a lot of this poop and makes the environment cleaner and healthier for the fish.
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To clean the gravel you use a basic model gravel cleaner like the one in the following link. It allows you to remove the gunk from the substrate without disturbing the tank. You can get a long piece of plastic hose or garden hose and attach it to the gravel cleaner and run it out the door onto the lawn, or just drain it into buckets.
https://www.about-goldfish.com/aquarium-cleaning.html
To fill the tank back up, get a couple of clean buckets that have never been used for anything and write "FISH ONLY" on them. Use those buckets for the fish and nothing else.
Fill the bucket with tapwater and add enough dechlorinator for that bucket of water. If the bucket holds 10 litres then add enough dechlorinator for 10 litres. Then aerate the bucket of water for at least 5 minutes (preferably 30 minutes or more) to allow the dechlorinator enough time to come in contact with all of the chlorine/ chloramine in the water, and to let the water re-gas.
When water is under pressure (like in water pipes), the gasses can be forced out of them. Aerating the water allows the carbon dioxide (CO2), oxygen (O2), and nitrogen (N2) gasses to get back into the water in the correct amounts.
Once the bucket of water and dechlorinator has been mixed, carefully pour or syphon it into the tank.
For large volumes of water you can use a large plastic rubbish bin or even a wheelie bin, or a large plastic storage container. Fill it with water, add enough dechlorinator for that container of water (not the tank, just the container of water you are making up), aerate for a bit and then use a small water pump and some plastic hose to pump the water into the tank.
You can make a U shape from pvc pipe or black irrigation pipe (aka poly pipe) and this can hang over the side of the tank so you don't need to hold the hose while it fills up.
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If you are using 10 litre buckets and your dechlorinator's dose rate is 10ml per 100 litres of water, add 1 ml of dechlorinator to each bucket of tap water. Aerate it and then add that bucket of water to the tank.
If you make up the water before doing the water change, it can aerate and dechlorinate while you are draining and gravel cleaning the tank.
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Most fish health issues are caused by poor water quality. The following link has information about what to do if your fish get sick. It's long and boring but worth knowing. I recommend printing it out and reading it in bed to help fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/
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If you post some pictures of the tank, we might be able to offer some advice on its set up. If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
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If you can tell us about your filter and how you clean it, we can offer advice on that too.
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Post a list of the fish you have, and have had and we can go through it and make suggestions on compatibility.
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What is the GH (general hardness) and pH of your water supply. This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
