Black Molly Acting Strange

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Aquarium Advice Apprentice
Jul 24, 2006
I have a 29 gallon tank and right now there are 5 rasboras, 4 threadfin rainbows, 4 neon tetras, one black molly and one fish I'm not completely sure about, on the reciept it said Platy. We recently lost the other Black Molly for unknown reasons and one rasbora before that. We did a partial water change afterwards. The water levels are all normal.

Anyway, my Black Molly has a weird obsession. He tends to rest around the heater. He curls himself around there for about a half hour everytime it turns on. The water temperature is around 84 degrees. I think he may have burnt himself or something? It started about 4 days ago. He was laying on the bottom of the tank very sluggish and his gills were moving very quickly. We thought he was on his way out, but he pulled through. Now it seems he has lost some of his control of his tail fin. He can still swim, but he swims almost vertically. He is still able to eat, so now he's acting normal besides the swimming and resting at the bottom frequently. Was he perhaps burnt by the heater since he sits wrapped around it? He pulled through, he's one of my favorite fish, always swimming to the side to great me when I pass by, so I'm glad he made it. I'm just confused at what happened.
Does anyone know what this might be? He's still swimming weird and the very tip of his tail looks sort of thinner than it should, almost see through.

And on a side note, I forgot to say I also have hatchet fish in the tank, I forgot to mention that in the previous post.
First thing I would do is a 50% water change. Then turn down the heat slowly. The tank is too hot for the fish. Mollies are fine around 78F. At that high a temp the oxygen in the water is less and your fish is probably need more oxygen. Until you get the temps lowered, keep the water a bit below the frame line to create more splash and see if the molly improves. He may be suffering other symptoms from water quality as well. What are your exact water parameters? Nitrite, ammonia and nitrate? How long has the tank been set up?
Thanks. We will do the 50% water change tomorrow. I just checked the heater, it's set at 77 degrees, but the thermometer is at 80 right now. I don't know how good this thermometer is, I'll probably end up buying a new one soon. The tank has been set up for about a month and a half. The water paerameters are: Nitrate 30 ppm; Nitrite 0 ppm; total hardiness 120 ppm; total alkalinity 0 ppm; ph 6.4; Ammonia 1 ppm. Now one of our Hatchet fish is acting strange. He is able to swim, but glides down to the bottom when he's near the filter, then swims back up to the top. I don't know if this is related, but one of the plants in the tank came with a small snail. A while ago, we had a small fish bowl and I remember once we noticed the snail, the fish got sick. Could this be related?
Ammonia is too high, ammonia should be 0ppm. You need to do that water change to keep the ammonia as close to 0ppm as possible. Heater settings can be notoriously off. I rely on the thermometer to guage temp of the tank. On most of my tanks the heaters are set a few degrees below the actual temp.
We did the 50% water change today. Now we have a problem. One of the hatchet fish is missing, the one that was acting strange. Nowhere to be found. I noticed I couldn't find him a few days ago, but he never emerged. I checked underneat all of the ornaments and gravel and in the filter, but I simply can not find him at all. Does anyone have any possible explanations to this?
Only two things I can think of:

1. He died, and the other fish picked at him enough where there wasn't much left by the time you went looking for him.

2. He jumped out of the tank. Not sure if hatchets are known as jumpers, but I had a couple fish go missing on me, turns out months later I was dusting around the fish tank and found two shriveled up fish sitting on the ground in the 1/2" of space between the tank stand and the wall. (Only opening they could have jumped out of was about a 1" of space where the water from my HOB filter comes back into the tank. For a fish that's a jumper, that's enough!)

P.S. -- I agree with what's already been said, if there is a discrepancy between what your heater says and what your thermometer says, trust the thermometer. I have a really nice, expensive all-stainless steel heater in my main tank, in order to keep the tank temp at about 78 I have to have the heater set for 73.5. Go figure.
I checked around the tank, no fish I can see. I guess it's just a mystery, then. This morning, I checked on the Molly and he is completely back to normal. The water change helped a lot. We also took out the plants temporarily because they were starting to grow something on them. We'll go soon to replace them.
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