secondary tank...?

[mkr]

Hello.

I have a healthy 90 gallon reef tank that's been running for a year now using the Berlin method. Livestock is happy and healthy and I've had no problems (diseases, nuisance algae, aiptasia...) at all.

I have considered for a long time to attach a secondary tank to my main sump (preferrably about 60 gallons, maybe 70). One of my "reef buddies" has a small lion fish he rescued from being flushed. He wants me to adopt it since it's now living in his sump.

So now I have an excuse to start a secondary tank. My plan is to keep the lion a year or longer, perhaps finding a new home for it.

Since I always wanted a cowfish, I will now have a tank running which could host it. BUT it being connected with my main reef concerns me. Even though it would be pretty much alone, the thought of it dying off or getting stressed for some strange reason taking my entire precious reef system with it does not appeal to me at all.

I also consider trying a more all natural approach this time, since I will now run a species tank rather than a full blown reef with various highly delicate species and a larger bioload.

This would be a "fish only" tank - with quality live DSB and loads of live rock - *possibly* running without a skimmer. Any ideas, comments on this would be most welcome... In case I decide to run a seperate system, I will implement a sump. I have looked at mud analysis, and I must say I'm not sure yet I will go for that...