Fish swimming to top and nasty bug

Question
I'm brand new to owning an aquarium. It's a 20 gallon freshwater with hornwart plants. I have had it up and running for two months. I currently have 1 albino pleco, 3 day-glo danios, 3 cherry barbs, 3 platys, 2 mollies and 3 cardinal tetras.

I performed a water change yesterday and also rinsed off my filter (Whisper 20-40 HOB) as it had so much debris on it, it was clogging the water flow.

This morning, I noticed all of the fish were all at the top of the tank at the water surface which is very odd. I also noticed my pleco kept swimming to the surface, smaking the the hood and going back to his hiding spot. This is VERY unusal for him. I have never seen him do this before. He always hides during the day and only comes out at night for his Algea wafer.

I thought maybe it was low oxegyn so I draned the tank a little to give the HOB filter more conatct with the air.

During this time I tested the water and found that the Nitrite had increased to 1.0. I did a 1/3 water change and the fish all started swimming around in the normal areas again.

Now, about 4 hours later, I notice the platy's and the mollies are starting to swim towards the top again. The Nitrite and Nitrate are testing fine (Nitrite at .25). Should I perform another water change? Am I missing something?

Also, during my water change, a very strange looking insect swam to the top of the tank. I have no idea what it is so I took it out of the tank. It's about an inch long, big bug eye's, 6 legs, 3 fin's on the end ....I can send a picture but dont know where to send it to.

Can you help?


Answer
Hi Angie:  It sounds like you have been doing the right thing... the fish were suffering from toxemia which occurs when the water chemistry spikes.  The bug sounds like an anchor worm and you should dispose of it and keep your eye out for other worms.  It may have been in the hornwort.  If you ammonia is at zero then I would do a slight water change.  If your ammonia is elevated then I would clean the gravel and add beneficial bacteria to the water.  I would do partial water changes until the nitrite is within lower ranges.  Almost anytime the fish swim at the top of the tank it is almost always a water chemistry issue and you should do at least a 30% water change.  Keep me posted... dave