Yeah man... i've had the same problem crop up here and there over the years and there's never an explanation I can find! My tanks are properly cycled... in the beginning someone had told me it was possibly an algae-bloom and for anyone considering it NEVER use algae-fix it killed my fish... quick fixes are never the answer... And it turns out it wasn't algae.
Sometimes without any explanation I find my tanks get a little bit cloudy but it almost always clears up within a few days maybe a week. I figure it's probably just some kind of bacterial bloom for whatever reason and I don't worry about it. Check for phosphates, water hardness etc. if everything is in order you could potentially do more harm changing out so much water at once. You don't want to accidently take out too much of your bacteria and once the bacteria levels are unbalanced you are not only prolonging the amount of time your tank will look cloudy because the bacteria are freaking out but you stand a chance in developing unwanted ammonia or nitrite levels in your tank that can kill or make your fishies sick.
If your tank was cycled before the water change, then you are most likely going through a minI cycle. Somehow, you have killed off some of your bacteria, and it's playing catch up right now. If it's something that happens every week when you do a water change, you might want to consider adding another filter to your tank. I have a 55 gal. with 2 angels and a 6 inch pleco in it right now, and when it had 5 angels in it, the water would turn cloudy the next day after a water change. I was running 2 sponge filters on it and added a 3rd sponge filter. Now it stays clear, and did when it had more fish in it.
We are struggling with the same issue. We cycled our tank back in December/January. It started looking grungy and we noticed algae starting to show up on everything. Then my Yoyo Loach started hiding and not eating so we figured we HAD to do something. We did a complete water change (emptied the tank, scrubbed everything down (no soap), rinsed the gravel). The fish were happy and the water was clear...for a few days. Then it started getting cloudy, more each day, until we couldn't even see the back of the tank. My Yoyo went back to hiding/not eating. We brought a picture into the LFS who told us it was likely an algae bloom and recommended Algone and a phosphate-eliminating pad (in place of the carbon filter) - said to give it a week.
Well, within 3 days my poor CoryCat died We did another water change and, again, the water was clear for about a day and then started to gradually get cloudy. The Yoyo Loach had died at some point as well (we found him in a rock when we did the water change out).
I don't know what to do at this point. We've been testing with the API kit and all of the levels have been fine. We also have one of those Ammonia indicating charts in the tank to make sure it doesn't peak at any time. Haven't had that problem.
If it is a bacteria bloom, how do we fix it? I'm afraid if we wait it out again we'll have the same problem we did the first time - the water will get so cloudy we won't be able to see through it and the fish will get sick and die.
By taking the tank down like that, doesn't it make the cycle begin all over? with the fish in there, would/could you put one of the Bio-sphere bacteria starter things in? Wait a week and then maybe do half a water change. just a thought or two good luck
If your tank is fairly new or going through its cycle then its going to get cloudy. That is just bacteria blooming.
If your tank has been established for over a month or more, and it is cloudy then all you need to do is one of two things.
1). Get another filter, it will keep your water clean of left over food and fish poop. Which will clear up your water.
2). Do more frequent water changes. 20 - 30% is all you need to do, more than that in an established tank and you could start a mini-cycle. If your doing 1 water change a week, then bump it up to 2 water changes a week.
Whatever you do don't go adding all these different chemicals to your water, especially if its already cycled. All your going to do is screw up the chemical balance and throw it into a mini-cycle or maybe even a full blown cycle.