Alright, I finally found your thread! Let's get down to business.
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First and foremost, I would have a moderator move this thread to the Other Monster Fish sub-forum of the Freshwater section. You will doubtlessly get more replies there. I find them to be more of a freshwater fish than brackish. In my years keeping them I have never needed more than a bit of salt in the first few weeks before I was comfortable keeping them for years in straight freshwater.
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They are reported to be frequent sand-sifters. Will very fine gravel be an okay substitute?
I have kept Dorm. Mac. on literally all types of substrate. I find that since they spend most of their time with their fat stomachs pressed to the bottom that only the finest gravel should be used to prevent scratching. If you decide to go with gravel, make sure that he isn't eating it when he lunges for food. Spitting it out is fine, obviously. I've never experienced a problem with sand-sifting, but I can see where it might occur. They are territorial towards their cave (which they
definitely need to not be stressed) and can end up causing some sand storms while making himself comfortable or chasing someone away. I use Caribsea Eco-Complete. Most of my fish like the soft, natural bottom that is also dark.
It should be noted that D. Mac. will devour most plants in an aquarium setting, including plastic ones. I've had minor luck with Anubias, who can withstand the constant tugging, but even if the plant is undamaged expect to have to continuously re-root it unless you place it high in the tank.
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My plan is to keep him in at least light brackish until he is a little bigger and stronger, then acclimate him back to fresh for a long time.
Again, I feel that salt will quickly become unnecessary, if it is at all to begin with. These are tough fish that acclimate quickly. Mine has been in a freshwater setting for over a year now.
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Your ID research seems correct. I don't often see such vibrant face striping on a D. Mac. but everything else fits the bill. I think those stripes will quickly fade as he grows, as these Gobies are much more beautiful when juvenile. D. Mac. seems to be the most commonly imported species.
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How much luck have people had in weaning these suckers onto pellets?
This is where I'm a little confused. This fish should eat everything it can fit in its mouth. Plants are part of its diet in most cases. The ones I have had took flakes when young, and I am certain my big boy would still take flakes if he ever came to the surface anymore. He catches sinking pellets in his mouth, and has been doing so since I got him. No weaning should be necessary. His other favorites are live bloodworms and frozen krill. Feeder fish are not recommended as this fish is a glutton who could do without the excess fatty foods. There is also the risk of parasites when it comes to feeders, obviously. Mine used to take guppies when it was smaller but now it prefers waiting to be fed rather than compete with the bigger predators for feeders.
Tankmates too small will likely be eaten, or killed trying to be eaten. To date, here are some tankmates that I have lost to D. Mac. when I was inexperienced with them.
At four inches, one of my D. Mac. ate a Leopard Ctenopoma "Leaf Fish" that was almost the same size. The goby got spined from the fins and got the fish stuck in his mouth for about four days before he could spit the carcass out. A light infection occurred on his lip that water changes and salt cleared up within days.
At five inches, one of mine ate a four inch albino rainbow shark. It grabbed him about five minutes after dropping it in the tank.
A four inch electric catfish was placed in my tank and had to be removed within a week. All of my predators learned to avoid it, but my D. Mac. would bite at it whenever it came out of hiding and receive a nasty shock as payment. He never learned not to try and eat it. He sustained no injuries but likely would have tried eating the catfish until shocked to death.
Clown loaches that were bigger than the Goby were harrassed and injured and quickly removed.
At six inches, my current D. Mac. caught a five inch Silver Needlenose at night after spending almost six months of co-habiting a 150 gallon tank with it. The shape and size of the Needlefish made for an awkward eating, but he eventually devoured most of it and spit the head out a day later.
At seven inches, the same Goby ate a six inch red-tailed barracuda and tried going after a second one before I intervened and removed the new fish.
Jade Gobies love to eat, and their big mouths mean they will try to eat pretty much anything. They don't mind having to suck on a fish the same size as them for days before it breaks down and they can swallow it. Of course, most of the above fish were experimental tankmates or were the result of me being inexperienced with Sleeper Gobies, so tankmates shouldn't be a problem if you plan accordingly.
Some tankmates I recommend:
Datnoids, who are too quick for the D. Mac. and will grow alongside them until they aren't considered food. Mine shakes his head at my Datnoid sometimes as a territorial display to scare him off, but it doesn't work.
Arowanas, but they must be larger than the Jade Goby or close to it. Mine was very interested in a juvenile Black Arowana. I was worried there might be an incident, but the Arowana grew too fast. I also keep a large African Arowana in my tank.
Bichirs, who might be chased away from the Gobies cave, but nothing more. I keep 2x Delhezi, 1x Ornate and 1x Endli with my big D. Mac. No aggression and no trying to eat them. I suppose the armor is a turn-off.
Big Catfish and Plecos, have never been a problem for me. I used to keep a Common Pleco with my Goby and it did eventually die, but I don't think the Goby had anything to do with it. My Marbled Pim catfish has no issues with the Goby, and they are both approaching ten inches.
"True Gar", aka Florida, Spotted, Tropical, etc. They MUST be considerably bigger than the Goby when introduced to the tank or there will be a problem.
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I would not recommend keeping Cichlids with your Jade Goby, but it is definitely possible. Despite being a lawn mower and a carnivore, the D. Mac. is actually quite the coward. He will hide from fish he is unsure about and try all sorts of defensive flaring to scare them away. I find that Cichlids love to bully Sleeper Gobies and usually aren't compatible for this reason. However, I have kept certain Pike Cichlids with them to some degree of success. There is still some bullying and chasing, but nothing severe. Make sure they are big pikes if you do, such as Crenicichla Johanna.
Also - no silvery fish unless they are much bigger!
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Provided that you upgrade accordingly, expect your Jade Goby to probably max out at around ten to twelve inches and grow fairly quickly in the first year (at least to seven inches) and then slow down. I wouldn't expect that hump back to slow him down too much, but maybe a little. It doesn't look too bad in your pictures.
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Jade Gobies have varying degrees of aggression. Most people say that they are softies with big appetites, but some of mine have been decidedly mean and nasty. So you're going to have to learn from your own, because it seems that each one is different.
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If you have any other questions, shoot me another PM. Please remember that since there is a lack of knowledge about this fish online, most of my information is coming strictly from my own experience with the D. Mac., which does span several years. I think I know the fish fairly well, but they can be quite the surprise just when you think you've got them figured out.
