I've been using metronidazole in a Q/Tx tank with a handful of invertebrates and can share some observations.
Treatment with metronidazole (both in the water and as an in-feed preparation) has been on-going for about a month with no observable (short term) ill effects on: macro algae (3 species), zooanthids (1 species), mushroom corals (1 species), coralline algae on some live rock and more recently, cleaner shrimp (n=2), hermit crabs (n=5, very small), and biological filtration.
A little more backgroundââ"šÂ¬Ã‚¦
I was treating in the water for a little over two weeks, every 3 days (with 1/4 water change prior to each addition of metro), at 6 mg/L (UV off) to treat a Yellowtail Angelfish that had been in my Q tank for a couple of weeks. When I first got the fish it appeared clear, but after a week a few Cryptocaryon spots showed up. My initial reaction was to begin treating the water with metronidazole. The infection was mild, and over the course of two weeks of in-water treatment the infection got cyclically worse (in what appeared to be 3-4 day cycles). I was using a formulation with 50% metronidazole/50% potassium sulfate. However, the fish appeared fine (other than the spots) and didn't show any overt signs of stress. At its worse point, I would have rated the infection as mild to moderate.
I then switched gears by: 1) raising the temperature from 76 to 80-81C, 2) adding two cleaner shrimp, 3) turning my UV unit back on - with a new bulb, 4) switching from adding metronidazole to the water to an in-feed treatment. The feed was prepared with metronidazole (pure form) added at a rate of 250 mg/100g of feed, which consisted of: fresh clams, mussels, squid, pieces of ON seaweed selects and a vitamin supplement (chopped fine in a blender), bound with gelatine and then frozen. I have been feeding this to the fish 2x per day for the last 8 days.
From my close visual inspections, the fish appears to be spot free for about 6 days now. There were a couple lingering spots on the eyes initially, but these too have disappeared. It will be interesting to see if the spots come back. If they do, my next move will be to set up a separate tank and to treat the fish with either hypo or copper. For a variety of practical reasons I have been avoiding this, and am very interested in alternative treatment options.
Obviously, I have no idea what worked here: but do know that the shrimp had a major impact. They were on my fish, both of them, within 10 minutes of being introduced to the tank. In fact, the fish was soliciting the shrimps in their bags as they were acclimating to the water. The fish then spent a lot of time with the shrimp over the next 2 days. The fish continues to spend time with the shrimp, but more casually. However, any discussion on which of the above factors was responsible (metro in water/shrimp/metro in feed/UV/temperature - all of the above) would be pure speculation. This fish was an unusual find (not often stocked in LFS around here) and given that is cost me well over $100, I didn't have any replicates or controls! It could be argued that if I had done nothing the fish may have got better by it's own devices - but not in this case as the infection appeared to be getting worse with each round of infection.
Both the shrimps and the hermits crabs (brought in with the shrimp) have been eating the medicated feed. I also suspect small crumbs have made their way into the zoos and mushroom corals - no ill effects have been noted in any of the invertebrates, or the water quality (which may be suggestive of stress in the bacteria in my filter). The fish has been doing great and eats like a champ (but has done so since day one). Having said that, metronidazole could have a deleterious effect on bacteria due to its mode of action but I'm not aware of any studies to this effect (i.e., on bacterial fauna found in fish tanks/filtration systems (FW or SW)). It is very possible that over prolonged use (especially if added to the water) it could stress the bacteria in your filter/live rock, or even zooanthellae colonizing certain invertebrates.
I will keep monitoring the fish's and invertebrates' progress and update this thread with any helpful observations.
