I set up a new 20gal tank a few weeks ago and it has been a nightmare ever since! I decided to go with a sand bottom for the first time ever and used aquarium sand from the pet store. That was a huge enough pain in the ass, but it has just gotten worse from there on out.

20 gal tall
Marineland 30gal HOB filter
Pain in the butt bubble wall
Natural rock
Three fake plants
Two moss balls
3 danios
2 gold dojo loaches
1 apple snail
4 julii cories
1 mickey mouse platy


So here's what's going on right now. ^

I thoroughly cleaned this tank two nights ago, Sunday evening. That night, and Monday morning the water was clear. Now, Tuesday night, it looks like this. Super cloudy. I don't know why! The filter appears to be working, though I've been having problems with it. They are eating the food we give them, it's not building up on the bottom.

Here's the story.
Ever since I started this tank with sand in the bottom, my first time doing a sand bottom, I have had problems with my filter. I have read up on what sort of filters to use in a sand aquarium and I all I can find is conflicting information which is irritating. I have a HOB filter that I've had for several months. It use to be in my 30 gallon tank, but moved it to the 20 when I set it up and got a bigger filter for my 30 gal tank. Within days of setting up the new 20 gallon with sand, the filter started making this awful noise. It sounded like a food processor grinding away. I assumed there was sand in the impeller mechanism. I took the filter apart, cleaned it out really well, really didn't find sand in it, replaced the filter media, and reassembled it. No difference, still loud and grinding and awful sounding. The next day I bought a sponge filter cover to put over the bottom of the intake tube to help filter out sand before it got to the impeller. It worked, it quieted the filter, no more grinding, sounded good. But nothing was being filtered through the actual filter media! The sponge cover was catching all the poop and food debris, and in a couple days, no water was being sucked up into the HOB filter, and there was barely even a trickle of water coming out of it. Terrible! That was last Tuesday, Jan 29th, and so we cleaned the tank, took apart and reassembled the filter and got it all tidied up. There was a lot of food build up on the bottom at this time, and I had a hard time cleaning it all up. The apple snail produces a lot of slime in this sand-bottom tank (something it didn't use to do in the 10gal gravel bottom it was in before), and it gets really heaving with crud in it and my siphon doesn't suck it all up. Hard to clean and Frustrating! Do you recommend a different kind of siphon besides the tube kind?

Friday, three days after I cleaned the tank all my danios (4 at the time) got fungus on their fins. At noon they were fine, by 5pm they looked like hell, 9pm was worse. I don't know why, but there was still a lot of food and poop buildup on the bottom from it being so hard to suck up when I cleaned it just days before. So I immediately treat them with Pimafix. It's what I have. The next morning they looked a little worse, and now it was spread to my platy. I continued to treat with Pimafix, and also salted the tank some.
Come Sunday the danios were looking better, except for one which appeared to have fungus in its right gill, but no where else. When I got home from work Sunday afternoon, a blue apple snail was floating dead, leaving its yellow snail friend alone. I only had the blue one a couple weeks. The tank was just awful and I had to clean it, just 5 days after I had cleaned it before. It was bad, like I hadn't done it just days before. I did at least a 50% water change with the clean and made sure to take extra care and get every bit of left over food and poop out of the tank and clean all the decor really well. Then I fluffed the sand to make sure there wasn't any trapped air, and there wasn't. Refilled the tank, treated the water, put the decor back in, plugged up the filter, everything looked great except the danio with gill fungus. The water was crystal clear and pretty.
Monday, danios look a lot better with fin fungus, but the one with gill fungus looked worse. Still treating with Pimafix daily, and this point included an antibiotic. The platy still had a little fungus on her, but was looking better. Monday evening the water is cloudy. What the heck? Is it from the antibiotic?
Tuesday, the danio with gill fungus looks terrible, the fungus is huge, hasn't been eating the past two day, so I decide to quarantine to a small container on kitchen counter. Within a matter of hours she dies. The other danios in the tank look all better, back to normal with no fungus. Platy is looking a bit better too, but not all gone yet. But the water is so so cloudy murky YUCK! WHY?!!?!

Why is this tank such a nightmare? I have a 30 gallon that I've had for years with big clown loaches and an ancient 5.5 year old tiger barb. They're fine! Never and problems! Why is this 20gallon tank awful? Is it the sand? The filter? Why are my fish sick so suddenly? Did the antibiotic (which i used one dose of) make the water all cloudy? How can I fix it? I've had fish my whole life, and have been keeping them myself all my adult life. I'm not stupid, but I'm SO lost on this one!

UPDATE 2/6/13 Wed: I did a 50% water change and it looks maybe 50% better, but not GOOD.