There’s a lot of misinformation out there around water changes. I’ve seen it many times; people who post online asking what people’s regimes are, and they receive a wide range of responses. I disagree with a lot of the answers, and feel that I have found a tried and true system that not only works but has the aim of not wasting water or time.
If you’re looking for a golden rule, unfortunately there isn’t one. It does depend on how cycled your aquarium is. Ideally you’ve cycled you aquarium properly and let it get really nasty and built up some grime in the filter. That way, things can only improve, and it’s unlikely the bacteria won’t be able to handle whatever waste level your fish are going to create on a weekly basis. Once your levels are good (maybe ammonia only spiking a tiny bit in a two week period) then you know you are ready to settle into a cleaning routine where you don’t need to be babysitting the tank so much.
Generally, I do a 20% water change every week-10 days. A little under half of that is evaporation. 20% is not an arbitrary amount. It’s how much I’ve found that it takes to get my tank actually clean. Here’s my guidelines to find that sweet spot for yourself.
Focus on getting it clean: I think sometimes people forget why the water quality may be poor in the tank, it’s not because you aren’t changing enough water, it’s because you aren’t doing a good enough job cleaning. The gravel is really where the harmful ammonia is building up. That mixed with uneaten food and plant matter, is all going to be breaking down. It may not be as visible as you think. If you have gravel then it probably is sinking down where you can’t see it. This means you need to get deeper in your gravel, at least a couple centimetres. You may not realize but your syphon may not be capable of getting that deep if it is just a straight cut tube. Sam’s aquariumz offers a water exchange system with a cut out design that allows for more debris to suck in. Also with a better tool you’ll have a slower flow, you’ll take out less water and get it cleaner. You don’t need to get it 100% perfect, just keep an eye on how low the level is getting and use your judgement on how much water you are taking out. Over time you’ll see it usually takes around the same amount, and is usually 10-25% percent. If it takes any longer then something else is probably going wrong, either overfeeding or overcrowding or not enough bottom feeders. At that point you really need to evaluate. Have you just not been thorough enough and it will take some time to get to par? Or are there other signs of trouble in the tank like fish loss or levels that aren’t consistent? You need to think hard about why your tank is getting so dirty because a good tank truly doesn’t need cleaning that much.
You should never just be aimlessly sucking water out of the tank. I think it’s a waste of water. The aquarium hobby has an environmental impact. We won’t always live in a world where we have unlimited access to clean water and you need to take that seriously. We live in a privileged society and the safety and abundance of water sources hangs in the balance of our actions.
It’s important to note what kind of things are normal, and you shouldn’t panic and do a huge water change. One of these things is cloudy water. This means you’ve had a bacteria bloom. They are most likely eating up some excessive nutrient that built up in the tank. So this plume is actually helping you and won’t hurt your fish. I would just do a 15% change on the first day and add some acuclear. This will take a couple days but should clear it up. Similarly a big algae ploom is going to mean the same. Some nutrient is thriving it, but there’s no need to panic and take out all the water. These things happen in most tanks as they tank is doing a long term cycle and different types of bacteria establish. It’s unsightly but turning the lights off will probably do more for you then doing a huge water change. You just need yo be patient and let it run its course. The only time you need to do a 50% change is if there is a huge spike of ammonia and nitrites. Once again, this won’t address the cause but will just keep your fish alive in the mean time.