The Brackish Tank

Mama Mia that's a big ol tank! Let's see if I can answer your questions/statements in order...

As for other suggestions, I have three:

My blog has caresheets for basically any brackish fish you are likely to find in the trade commonly or uncommonly. Take a browse and if you have any questions, let me know!

Well - that is admittedly a bit of a dilemma. I'd still consider trying pond or ramshorn snails (I also don't think F8s really possess the ability to damage Nerites) but barring that, your only real choice is to outcompete the algae with live plants. Do you have any plants currently?

My understanding is that Peacock Gudgeons don't have long term brackish tolerance. Please keep in freshwater only! If you want a brackish sleeper goby, look into the related Empire Gudgeon.

There are two answers here - a general and a specific. From a general standpoint, I think many marine fish possibly have more tolerance for hyposalinity than we give credit for. At least one significant, well respected author and aquarist, Lance Ichinotsubo, states he keeps his fish only marine tanks at 1.010 over the long term with great results. I couldn't say myself how effective or ethical this is but I imagine it's an topic that is vastly underexplored.

Specifically, there are a good handful of marine species that can tolerate this range. Of course there are the brackish water staples like Scats, Monos, Tete Sea/Silver Shark Catfish, and any number of flounders/soles, killifish, and livebearers that make their way into full salinity for long stretches of time. But there are also several Puffer species, not to mention Needlefish, a few Stonefish, Lionfish, Zebra and Peacock Bennies, Batfish/Spadefish (Platax spp being well known), Stripies, Barramundis, several Snapper species, Sergeant Majors, and a few Waspfish. And these are only the species that appear in the aquarium trade! I suspect several hundred species can make that transition.

My F8 puffer recently passed, sadly, but I have some advice for you still that hopefully you find helpful.

First of all - I'd suggest going with 1.002 or 1.003 instead of 1.005 if you want plants (vice macroalgae). It'll open up your options significantly. Java Ferns, Anubias, Vallisneria, and Amazon Swords are your best bets if you do - though research suggests some species of Crypts, Hygrophila, and Bacopa can work too. If you are set on 1.005, look to Brookweed and Microswords, which can tolerate the higher salinity but are fussier with respect to lights and substrate. Java Ferns also anecdotally tolerate the 1.005 but I've heard reports of people keeping them alive much higher than that.

There are a few species of otherwise marine macroalgaes that can survive down to 1.005 - chaetomorpha and Ulva spp. Sea Lettuce come to mind. On the other side of the coin, Marimos usually tolerate up to 1.005 or so (there is anecdotal evidence to suggest it can tolerate much higher, but that hasn't been my experience).

I think this is a very old ask - sorry I've been lazy!

The thing about hermits is that they are all pretty much omnivorous to some degree and many species flat out should not be trusted with smaller tankmates. Calcinus laurentae and C. tibicen are pretty safe bets with a variety of tankmates, though. My favorite species, almost always reef safe, is Clibanarius tricolor. Very peaceful, very common and inexpensive. I'd look into those first.

I've heard a little - as much as I used to like thinking I was the authority on tumblr for brackish animals, I really only focused on those species that made their way into the trade commonly or uncommonly. But that was about 10 years ago and things evolved a little since.

So! About the extent of my current knowledge is that they are a very small species of nerites, closest in size to the Olive Nerites that share the same genus. They can apparently tolerate fresh, brackish, and salt water - I've seen some recommendations for keeping them in reef tanks - but if you are open to making a tank around them, I'd probably keep them in lower end brackish conditions (around say 1.005) with suitable tankmates. A little higher or lower is fine, but they don't need the added money required for full saltwater if you don't have the desire to keep saltwater fish. Otherwise, care is the same as with Olive Nerites.

While it does depend a little on what species you are keeping - the general idea is the same. It's much easier to make features like your sandbar hold if you use varying sizes of substrate. Some river rock, some crushed coral or aragonite, and some pool filter sand altogether will keep your sandbar from eroding much. If you are able to supplement with some live salt-tolerant marsh plants, all the better.

Next, I recommend using some decor features that will let the mudskippers get out of line of sight of any aggressive tankmates. They make some great looking fake mangrove roots that I highly recommend - glue some oyster shells to them to get a realistic looking oyster reef. Just avoid real mangroves as they get too big. Some floating plants, real or fake, are also beneficial.

Do you know what species you'll be keeping?

As far as I am aware, T. blythii is purely a freshwater species with no long term brackish suitability. I think if push came to shove it probably could withstand 1.003 for a long period of time (maybe indefinitely?), but there's a difference between surviving and thriving and I would not subject the species to those conditions for longer than is necessary, as with treating ich.

Oh, definitely. While I've heard anecdotal evidence suggesting Green Spotter Puffers and Monos do best with low salinity as young, increasing the salt with age, Scats are truly adapted for marine life. They can live in full saltwater (1.025ish) from the start. I have personally kept young Scats in saltwater with no ill effects. No worries!

In a 29? As you've noted the tank is too small for much more than fingerling versions of the species you are keeping.

In a 125? You could likely fit a few chromides in there without issue. I think you'll find the aggression issues between the Scat and Mono will abate in this larger tank, but you may also decide that two more monos ate in your future. At 1.015 with a protein skimmer I think it'd work.

Well I really wish I knew much about shrimp biology - I know less about it than I do fish biology (which is admittedly limited. I am a hobbyist who enjoys learning about fish and has done a LOT of research, but still just a hobbyist). That said, Neale Monks literally wrote THE definitive book on brackish water fish tanks and cleared the Amano as safe for long term keeping in as much as 1.005. If anybody judges you for this decision, you can quote "Brackish-Water Fishes" as justification and ask them what professional source from a reputable source can definitively state they cannot tolerate this low level of salinity.

Lets do the easy answer first then the convoluted explanation next! As far as I know, the Moon Wrasse is not found in brackish areas. I'm pretty sure it is only found in and around reefs (but hey I might be wrong there).

The convoluted part is that salinity tolerance for fish is somewhat on the fluid side. I remember reading about a very prominent and well experienced aquarist who stated that he kept all FOWLR tanks for his clients at 1.012, though on paper all were saltwater fish. He reported they all lived healthy lives. But anecdotal evidence is weird like that - just because somebody reports success doesn't make it the best for everybody with all fish in all situations. Until somebody does an expansive experiment on the subject, I think it's best to treat this variable as an unknown and default to proven safe environmental conditions.

Well let's start with fish tankmates... it depends on your GSP! Some can be very aggressive and won't do well with normal tankmates. These guys will need other pushy tankmates like other (aggressive) GSPs, maybe Dascyllus damsels (in 1.020 or higher). They might be harder to place. On the other end of the spectrum, my GSP is a fantastically peaceful guy who's never nipped anybody. I've kept him with Mollies, Scats, Killifish, Damsels, Cardinal fish, Blue Eyes... not a peep. Realistically, I'd consider Mollies, Orange Chromides, brackish Fundulus killifish, and maybe some euryhaline gobies to be safe bets for the majority of GSPs. If you can find freshwater demoiselles, a species of black damsels with yellow tails sometimes sold as "assorted damselfish", they are great options.

As for plants - at 1.013 your only real options are seagrasses or mangroves and I wouldn't recommend either in a 55 gallon tank. Try looking at ornamental algaes- chaetomorpha, caulerpa, ulva, and a few others can survive salinities in the teens. If you live near some brackish areas, I recommend collecting some local species!

Honestly, I think your best bet at obtaining brackish plants to keep in an aquarium is to collect your own; there are remarkably few plants sold in the trade which can survive the salinity levels that mudskippers need long term. Brookweed and Microswords are your best bet, but both are somewhat demanding. Anecdotal evidence suggests marimos can (but I have had little luck with them above 1.003) and Chaetomorpha is another macroalgae option that can survive low salinities compared to full saltwater.

You may also decide to go with plastic, which is not a terrible option now either.