I've been fighting hydra in my nano shrimp tank for the last month or two, and finally have succeeded in eliminating them - hooray! So thought I'd post what I tried / failed / succeeded with in case it helps others....
Very small 10 litre shrimp only (RCS) tank, Amazonia soil, rocks covered in Christmas moss and floating salvinia on the surface. Soft tap water with minerals added to take to 7 dGH and pH around 7.0. No CO2, fertilised at recommended dose with complete Seachem range. Otherwise completely healthy, cycled and stable.
Unwittingly seeded this tank with hydra when I transferred plants from an existing tank, along with a host of other critters including white detritus worms (seen wriggling in water), small white flat worms (not planaria thankfully!), seed shrimp, cyclops, and who knows what else. Amazing what you find when you examine the tank with a torch and magnifying glass - aarrrggg! Subsequently spotted all the above in my fish tank, but that has fish so all kept under control. In the shrimp tank with nothing to eat them they were all getting out of hand. Initially I spotted bright green hydra on the glass (best way to see is back-lighting using torch with narrow beam with the tank lights off), but later I also found white ones too.
Not an option:
a) Panancur-C (fenbendazole canine dewormer) is safe and effective from what I've read online, but is a controlled drug here in Singapore and so only available on prescription from vets. To cut a long story short, they needed me to take a fish to them so they could take a stool sample to test and would then orally dose the fish with the medication there and then. Gave up trying to explain....
b) Add fish - the point of the tank is to be a fishless breeding tank for cherry shrimp
c) Break everything down and start again. And admit defeat?... never!
d) Feed less - I was not feeding this shrimp tank at all
, so over-feeding was not the cause and I could not feed any less.
First method - manual removal
You can definitely remove the odd hydra manually. I used a syringe to scrape the hydra off the glass and suck it up in one movement, but you do need a third hand to hold the torch. Important not to break up the hydra and leave any bits in tank as each will become a new mini-monster. The problem is that for every hydra you see on glass, there are 10-100 hiding elsewhere so you can at best control the numbers this way, but never eliminate them. The more I looked, the more I spotted. When I saw lots of really tiny hydra hidden between the grains of substrate (needed my glasses and two stacked magnifying glasses to see, being only a mm or so in size), I knew I could not ever win this battle. So...
Second method - Seachem Excel
I was already dosing Seachem Excel at the recommended dosage. Diluted in the tank water this had no effect on hydra, but spot treating did kill any hydra I directly hit with undiluted Excel. The problem is that the daily dose for my tank is about 1ml and that doesn't allow you to hit many hydra per day. I obviously didn't want to dose higher because of the shrimp, and actually I have now stopped using Excel at all in this tank because it didn't seem to do much to help my Christmas moss, and I've read that whilst long-term daily dosing might not be dangerous to shrimp, it might also not be optimal. So Excel is not useful really it eliminating hydra.
Third method - hydrogen peroxide
I found this video from Mark's Shrimp Tanks Youtube channel and other posts saying a similar approach could also be used to treat hydra, so decided to give it a go following his instructions:
The hydrogen peroxide had a positive effect on the tank. It eliminated hair algae on my most after the first attempt and initially seemed to have cleared the hydra from the glass. Shrimp seemed happy (super happy in fact during the treatment), but didn't do anything to the worms or other critters.
Next day, the hydra were back. So I tried dosing again a few days later at a slightly higher dose (2ml per 4.5 litres, rather than 1.5ml as Mark suggests in his video). I also tried to spot dose in the areas where i knew hydra lurked. I examined the tank each night with torch and magnifying glass, and did further spot treating of any hydra I found. I found that the HP was just as effective if diluted 50:50 with tank water - draw 1ml into small syringe and then draw up another 1 ml of tank water, then squirt through a blunt-ended needle. Any hydra I hit immediately shriveled up, bigger ones fizzed a bit, and they were gone.
However, next day there were always others and I didn't want to keep squirting HP in the tank every night to control. My conclusion is that hydrogen peroxide with kill hydra when they are directly squirted, even if further diluted with water. However the dilution within the general tank water is not enough to kill all hydra, especially those hidden in the substrate. So, again, I think this is a way to control hydra but is definitely not a complete cure. It is a good thing to do anyway for shrimp tanks though, as per the above video! Didn't seem to adversely affect my trumpet snails either.
Fourth method - "No Planaria" powder
With the lockdown here in SG easing I was able to get to a different LFS that had some "No Planaria" in stock. Removed my snail from the tank (Malaysian trumpet snail) along with as many snail babies as I could find (I'm happy to have some of these in the tank and my kids like spotting them rummaging around through the soil). I dosed following the instructions, but this is pretty hard given that you only need a very small amount, especially for a 10 litre tank. In the end, I bought a new set on mini weighing scales (down to 0.01g accuracy), weighed the recommended 1 spoonful dose and then divided this down to get the accurate weight to add to my tank. Dissolved this in water in a big syringe and then squirted around tank.
After less than 1 hour, all hydra on the glass had dropped off - hooray! The next day I did the recommended 50% top-up dose to be sure, but I was unable to find any hydra anywhere in the tank. There were still detritus and flatworms on the glass though, but I wasn't surprised as I had read that it might not be effective against non-planaria worms. Oh well, at least the hydra were gone! I didn't do the recommended 25% top-up dose on day 3 as it was such a tiny amount, too small to measure properly, and there didn't seem to be any need.
So on day 3 I did a water change and added carbon to the filter to mop-up and remaining medication. There were no adverse effects on either shrimp or plants. Surprisingly it did not affect my snails that I hadn't managed to remove - maybe 5 or 10 tiny baby trumpet snails. These are still fine a few weeks on, so I wouldn't bother removing them next time. Maybe because they spend most of their time in the substrate they are protected? Other snail types might not be so lucky though I guess, so probably safest to remove. Also no adverse effect on bio-filter.
A few days later and all the worms seem to have pretty much gone, or at least I've not spotted any white wigglers in the water or white flat worms zooming across the glass over the past few weeks since treatment. So "No Planaria" seems effective against these detritus worms too, but it takes a few days to do it's job, not instant as per the hydra. The seed shrimp and cyclops type mini beasts were still there though, seemingly unaffected, and no big issue really. Actually pretty cool to watch.
Fifth method - adding fish
Ok, so I know I said that this was a shrimp only tank. But my LFS got in a shipment of tiny micro-rasbora ("Exclamation Mark Rasbora apparently) which are about 3/4" long. The guy I know there (an experienced shrimp keeper) assured me they would be pretty much shrimp safe, and given the amount of hiding places in all my Christmas moss, I thought it worth a try. I'm only concerned about juvenile shrimp of course as the adults are actually bigger and meaner than the fish!
These fish are great!!! I have a small shoal of 10 or so, they're tiny and perfectly within scale in this small tank. They don't bother the shrimp at all and, after watching what / how they eat, I think even juvenile shrimp will be too small to fit in their mouths. They can just about eat one or two tiny grains of the Dr Bassleer Biofish-Food
I give them (0.5-0.8mm grain size according to the tub), but sometimes it takes them a couple of goes to swallow them. They much prefer the GlasGarten Baby Shrimp Food "dust" I feed the shrimp.
Since adding the fish, pretty much all the mini-beasts in the tank have gone - cyclops and seed shrimp as best I could identify under two stacked magnifying glasses (they were VERY small!). I suspect that all these food items swimming in the tank was the reason for the initial outbreak of hydra, so adding fish is definitely one of the best ways to eliminate (or at least effectively control hydra). They help starve the hydra by eating the mini-beasts, and some fish may also eat the hydra directly if they're hungry enough and so inclined. I think my micro-fish are to small to eat the hydra, but I'm sure that not having so much food would have helped keep them in check anyway.
You cannot completely eliminate hydra by manual removal alone (for every 1 you see, there are 10+ others that you don't, and breaking them up during clumsy removal can actually make the problem worst).
Excel kills hydra when they are directly squirted, but not effective in treating whole tank.
Hydrogen peroxide is more effective than Excel, safer, and has other benefits for the tank. Spot dosing works well for effective control, but wasn't able to eliminate hydrate even after repeated treatments and follow up spot treatments. But good thing to do in a shrimp tank and great for removing hair algae from moss!
Hydra outbreaks need food, but this is not necessarily because you overfeed the tank as many people will initially accuse you of. In a fishless tank there are probably lots of minibeasts that the hydra can feast upon, even if you don't feed the tank at all.
Adding the right fish is a great effective control without resorting to chemicals. Fish big enough to eat hydra directly are probably going to bother your shrimp, but micro fish will help by eating the mini-beasts and hence starving out the hydra.
Panacur-C (fenbendazole) apparently works great from what I have read online, also clearing out any worms from the tank, but it is hard to get in some countries.
"No Planaria" works great, completely shrimp safe at the recommended dose. It works within a hour or so on hydra so probably only need to do the initial day one dose and not bother with the day 2 (50%) and day 3 (25%) doses, although they will do no harm. Also gets rid of white wiggly detritus worms and non-planaria (rhabdocoela?) white flat worms seen on glass, but takes a few days to work and the 2nd and 3rd doses might be needed. Does not harm Malaysian trumpet snails, even tiny baby ones, but maybe because they are safe beneath substrate. Weeks later, there seems more than ever in the tank - good or bad thing depend on how you view your snails. Biggest issue is correct dosing as you need so little, especially for small tanks.
Basically there are only 2 ways to 100% eliminate hydra without breaking down tank and starting again:
1) Fenbendazole dewormer
2) No Planaria
Both are supposed to be shrimp safe, take your pick. I would say it is impossible to eliminate hydra without resorting to a chemical treatment, however having the right fish, restricting available food and spot treatment with HP will all help keep numbers in check... maybe. But you probably want them gone, not reduced, in a shrimp tank.
Hope this is helpful. It is only my experience though and all tanks are different, blah, blah, blah.... So please use your own judgement!