Ok so i have a 20 gallon bowed front fish tank. its taller than it is wide. With the filling sits probably closer to 16 gallons of water.

I had 4 fish and a algea eater in there for about 6-7 months before the algea eater died. I decided to buy a new one and another fish. Those both died in about 2 and i got a refund and chose to use a different store to purchase my next fish from. I bought 2 molly's and another algea eater, along with some algea chips to help him get started. 1 molly, the algea eater, and one of the original 4 have died since then, that was about 1 week ago.

So my tank currently has:
a black molly - about a year old
red tailed shark - about 1 year and 3 months old (hes about 4 inches long)
yellow molly - about 10 months
new dalmatian molly - 1 week

Any idea why this is happening? I've been doing 1/3 tank cleans every 2 weeks. I feed them pinch in morning of regular food, and 2x shrimp pellets that sink to bottom for the red tailed shark. He doesn't chase or torment the other fish or anything....

What's the temperature ?

Could it have recycled through the ammonia -> nitrite -> nitrate cycle?

Are you able to test the water levels for these? The API test kit is great for this.

Also, pH is important as well, if you have anything in the aquarium that isn't conventionally put in aquariums.

Temperature should be between 74 and 80 for most tropical fish.

Have you looked for ick, fungus, or anything else? It could be a parasite maybe.

Whenever I start having trouble I turn the temp on the tank up a couple degrees (gradually to 85) and add some aquarium salt.

What kind of filtration are you running? And do you have any live plants in it?

Oh, and with Plecos (algae eater). Many sold are caught wild and die for some reason shortly thereafter, usually because they don't eat well or never adapt. It is important to find them born in captivity. It is weird, but nobody will tell you that. Also, a common pleco is way too big for your tank...they start small but they hit 16 inches long...what you want to pick up is a Bristlenose Pleco, they cost around the same and only grow up to 5 or so inches...and they're cute as hell. I cut up cucumber and put it on a fork and put in in my aquarium and he will eat it til its gone.

Also, I've had made mollies fuck females to death, or at least overstress them until they died due to reproductive aggression. I have one molly, King Garrick, that has babies all over the community tank and a trail of broken hearts in his wake.

This post was edited by Skinned on Dec 17 2015 06:24am

I haven't touched it since i bought it, the heater just runs on the middle setting - almost 2.5 years now. My thermometer stopped sticking to wall so its not in there, i'll put it in there tonight and see what it reads.



I do not have a testing kit, but i'll take some water in to Petco, they test for free around here. I didn't see any Ick/Fungus.

Filtration is just the same pump/filter that came with the tank when i bought it. I'll look at brand tonight, but i use medium size filters that i replace once a month.

I know when my first fish died i took it into petco for a refund and they measured the water and he said it was a little high in nitrates, but he said it could have been from slight over feeding. I'll have to run some water tests by the sound of it and figure out what is going on.

I don't have any live plants that i put in there, but there is some algae growing off some of the ornaments in there that i let grow simply to get a solid foundation for the algae eaters. its not bad by any means.

seems weird to have all the fish dying but year old fish going strong. onl thing to do is make sure your water is perfect...and watch them and see if that shark is harrassing them, could be dying from stress?

Or check the fins to see if the shark is fin nipping if so could be stress....?

when you replace the filter are you replacing everything in the filter? Usually the filter will have 2 sets of pads, one that looks like cotton and one that looks like a sponge or a plastic scouring pad, you are only supposed to replace the one made of cotton, the sponge looking pad is used to accumulate bacteria that eat ammonia. you should only rinse the bacteria pad in old tank water to clean it (chlorine in tap water kills the bacteria).

if that isnt the problem and you get the thermometer and the temps are right then I would assume you got a bad set of fish from petco. they are pretty famous for buying sickly or inbred fish.

lastly it might be the waters gh, or kh is too low. when you say algae eater I am assuming a pleco which is completely fine in hard water with high ph gh and kh but from what I understand mollies wont do well in acidic water, that said plecos are fine in acidic water so this is most likely not the problem, but worth looking into.

Thanks for all the new inputs guys. After getting down to 3 fish i ended up doing full tank change with store bought water instead of water from apartment and took every part of filter/piece from inside tank and scrubbed it clean. I almost changed the rocks at bottom but decided against that extreme since i'm probably moving in couple months and will have to do it then. you get quite a few looks buying 20 gallons of water in jugs, thats for sure. been monitoring the water with test strips and what not. been doing very well. the nitrate levels are 2 colors down on the test strip bar now and 1 from being good. the water is getting more neutral (used to be very hard), and alkaline levels are way more neutral. 2 of the remaining fish had babies.....which were then eaten...circle of life i guess. I did have the heater set a little high because our heat was out through november, it was still in the green though for temp wise.

Going to monitor about 2 more weeks with current levels before trying to add 1 or 2. looks pretty stabilized though.

I would also like to get one ...

You have a shark in the fish tank is the problem