Dwarf Gouramis changing color after water change

We just did a complete take clean out. I put the fish in a smaller tank while a cleaned their home tank. All of the fish are swimming around exploring as usual. I was taking pictures when I noticed my male dwarf gouramis had changed colors. He has a dark blue stripe from his eye to the front tip of his bottom fin. Is this natural or something wrong with the water?

 

Welcome to Fishlore!

Can you take a pic of him?

Can you also give us a little more info? Tank size? What fish and how many are in the tank?

What are your water parameters (pH, temp, ammonia, nitrites, nitrates)? Did you clean their tank with anything? What did you do with the media in the filter?

 

Welcome to Fishlore!

Can you take a pic of him?

Can you also give us a little more info? Tank size? What fish and how many are in the tank?

What are your water parameters (pH, temp, ammonia, nitrites, nitrates)? Did you clean their tank with anything? What did you do with the media in the filter?

We have 3 goldfish, 5 catfish, and Gary the gouramis in a 25 gallon tank that is probably shy a gallon or 2 to allow for too air space. I don't have water specks. He has never had an issue before. This may have been a gradual change while he was in the other tank but it wasn't there a week ago. He isn't acting any different. Here is a picture of him and the catfish.

 

Can anyone else out there tell me what this is?

 

The blue is the mating-color for a DG. The catfish is a Corydoras aeneus albino. Goldfish are cold water fish and not compatible with (sub)tropical ones.
Do the gouramis have (floating)plants?

 

I got them together and was told they were fine. We have had him for over a year now. I have never put plants in the tank. I asked about plants and was told that they clog the filter. I thought they might help keep the water a little cleaner to balance out the goldfish being so dirty. I am changing a once a month filter every 2 weeks or so.