GloFish killing tank mates

Hi all! I'm new to the site, but have been keeping fish for years.

I am in the process of moving out of California, and am staying with my sister until I get my own place. I have ponds in CA, which are stocked with gambusia because I have a problem with raccoons, and smaller fish are harder for raccoons to catch. Gambusia are also very tough, tolerate a lot of conditions, and devour mosquito larvae. I caught 10 to bring with me, and set up a 25 gallon tank for them. The tank has rock formation decorations (plastic) and lots of live plants - amazon swords, java ferns, cardinal plants, and dwarf hairgrass. I decided on a Fluval U2 submersible filter, which is up toward the top of the tank to put the outflow jet where it will create a surface current. The filter is rated for a 30 gallon tank.

My sister loves neon tetras, so added 5 (which I later upped to 10 total). She also added 5 fancy male guppies, and I added 2 corydora cats and a black khuli loach to clean up any food off the bottom. I initially worried that the gambusia could be aggressive, but they show no signs of aggression toward anyone, and actually schooled with the neons. When I added the second batch of 5 neons, I also added 5 Starfire Red GloFish danios, which took to the top of the tank to school with the fancy male guppies in the filter current. Again, I watched for any signs of aggression, and everyone got along fine.

A couple days later my sister added 8 GloFish tetras (4 green, 4 purple) and 4 Galactic Purple danios. Then everything immediately went downhill, with fish disappearing or turning up with their tails eaten completely off. The culprits are the purple danios - they chase and attack everything, and in the course of three days, they have killed 10 fish. Several more have severely nipped tails and body wounds. Is this aggression normal?

I know the tank is most likely overstocked, but I'm meticulous about tank maintenance. Please don't beat me up on this point. I just need to know if this level of aggression is normal for GloFish danios so I can decide what kind of action to take.

Thanks. I'd never kept danios before, but have always liked them. We decided on a GloFish tank, but the largest kit available is a mere 10 gallon set. The kit is less expensive than buying the components on their own, so will be for the danios only. (No, I didn't move them yet, as I'm letting the tank cycle for 24 hours - unless it's recommended to go longer.)

Another thing I noticed was that the tank I set up for my monster tinfoil barb and reed fish was visible to the smaller community tank. Once I blocked line of sight, the community tank changed drastically.

Again, thanks for all the good info/advice.

I'm in the process of moving, and it will be a couple months before I can set up the large tanks I want.... The 10g GloKit was a temp solution for what appeared to be overly aggressive fish. Haven't put anything in it yet, but may tame the filter jet and relocate the 6 GloFish tetras. (We got a Nat Geo IFS20, and it shoots out a serious jet of water.) I'm staying with my sister until my house is ready, and she has space issues with another large tank.

I did set up a 46g bow for my "legal panfish" sized gold tinfoil barb ("Skippy") and ancient reed fish. (My sister will be keeping this tank as a community tank when I move into my place.) I have a small bichir, small bumblebee catfish, and dragonfish in the 46 too. Also 4 Denison barbs. Would you suggest moving the GloDanios to this tank, or would they become dinner?

I've read up on the GloFish and saw that they were genetically modified instead of dyed/dipped/painted, and that their original purpose was to monitor water quality. I don't have an issue with this, but Petsmart/Petco often label their fish with inaccurate info. My first bumblebee cat was listed as max size of 3 inches and needing a 20g tank. It grew to over 7 inches, and lived for over 20 years. They also listed tinfoil barbs as needing 20g minimum when I bought mine several years ago. (Now they are listed as 55g minimum, which still seems small.)

Also, I'm moving out of San Diego, and the tap water there is just nasty. (You can actually smell the algae in the tap water, especially after a heat wave.) I have long had an issue with foul smelling algae in my 20g aquarium, but don't want to toss good equipment. I have a large plastic stump decoration that is covered with "moss". Also have an "auxiliary filter" that sits on the back of the tank, which I believe is a Whisper or comparable Petco brand. The tank originally had an under gravel filter, which was the common type when I set it up. I added the Whisper to supplement filtration due to problems with the various air pumps for the UGF. (There is a long story in regards to the tank only being 20g. The short version is weight limits on the hardwood floor.) I plan on eventually setting up the 20g again as a small community, so would you recommend keeping the stump decoration, gravel, and filter to start the bio cycle? (It's currently still running, but there are no fish in the tank.) I plan on ditching the UGF and will probably go for a Fluval U2 submersible, and I plan on using live plants.

I know I appear absolutely clueless, but in my defense, my old 20g was at maximum capacity for the past 10 years (or longer), and space/time limitations kept me from setting up additional tanks.

I had an OSL! moment when I first saw them. Coming from Calif, I'd never seen them before, and something about neon pink/red was just too tempting. No, I would NEVER release aquarium fish into the wild, but some people unfortunately do. (Someone caught an 8lb pacu in the local reservoir several years back, and that wasn't the first one caught either.) And I agree, the neon colors of the GloFish pretty much guarantee they would be quickly picked off.

The aggression seems to have stopped now. I blocked the view of the "sea monster" tank, and the one particularly aggressive danio died. (Due to the swollen belly, I'm guessing egg-bound female.) Some of the injured fish went ahead and died despite my efforts to treat/save them. Getting ready to do a partial water change.

Overall, this has been quite the learning experience!