Mixing fish

[Michael Welber]

I'm wondering if one can add goldfish or Koi to a tank with tilapia in it. Will they get along? Or do battle? 

[Chip Pilkington]

I currently have a 1500L tank, as well as a pond with both Koi and Tilapia never had a problem. I've run into many folks on the forums with this combo - particulalry in Asia, where both Tilapia and Koi are popular. They do well on the same food and tolerate the same temp range, pH and all the rest.

I have never experienced any aggression or harmful behavior between the two.

[TCLynx]

I have one tank that has about 15 or so channel catfish, 30-40 bluegill, 1 Koi and a few goldfish.  A couple goldfish did die but I don't know what killed them.

[Michael Welber]

Which fish are more resilient? Tilapia, bluegill, or catfish? I have had a pretty bad die off of my tilapia and can't figure out why. I"m hoping an ichthyologist will come to take a look (a guy in the Keys) but I'm probably going to be starting over. If I'm going to buy fingerlings again I want to get some that do better. I know tilapia are supposed to be able to survive in anything but they didn't. 

[TCLynx]

I've found both bluegill and channel catfish to be fairly easy but poor water quality will cause you issues with them so you want to make sure your problem isn't water quality related before assuming they will do better.  Koi and goldfish can also be really robust (being carp) provided you don't get the cheapest feeder goldfish (those meant as food for other fish and are often the deformed genetic rejects that don't survive long anyway.)

[Michael Welber]

Will bluegill that are the same size as tilapia, eat the tilapia? I'm thinking about ordering 15 of each kind of fry. I don't have poor water quality but "something" is going on. I froze one of my dead ones in the hopes that an ichthyologist who lives in the Keys can look at it and tell me what's happening. 

[Chip Pilkington]

Hi Michael,

It would be wise to look into what killed the Tilapia. With the exception of low water temps, Tilapia are quite tolerent as to water quality and chemistry. Remember - just because it's clear, doesn't mean it's "good quality."

Whatever killed your Tilapia would most likely kill anything new into the system, particulalry species less tolerent.

Are you fully cycled?

[Michael Welber]

That's what's weird Chip. I was fully cycled two months ago. The fry did really well for about 45 days, eating and growing. Then fish started dying. Ammonia, nitrites and pH are all perfect. Some of it may have been due to debris coming back from the grow beds, making the water very dirty. I have put some passive filters on the return siphons and on the return pipe coming back into the grow bed and that has cleaned the water up considerably. Some of the problem was definitely due to bullying with one or two of the bigger fish killing smaller ones but that's not all of it. 

I'm using an old hot tub as my fish tank. I cleaned it thoroughly before filling it and, since it's made of fiberglass, there is no off-gassing. 

[Glenn]

About that oild hot tub.

Is there a chance that the many air and water jet holes were plugged with or contain any toxic materials, fittings, or ornaments? Coppper, zinc, and some sealants etc... are toxic to fish.

[Michael Welber]

We sealed all the holes with a product called 5200. It's a marine sealant. No copper, zinc. It's a polyurethane material. I was more concerned about plumbing glue but everyone says it's OK. 

[Glenn]

I've read that 3M's 5200 Marine Adhesive Sealant is great stuff for strength etc. but is NOT aquarium or reef safe for fish etc. Could the fish have nibbled on it any?

[Michael Welber]

Possible but not all of them certainly. I think a bigger problem would be that it would leach into the water. Good grief.