Yellowfin toxic goby

Where they come from

Yellowfin toxic gobies (Yongeichthys criniger) show up around mangroves, estuaries, and sheltered coastal flats in the Indo-Pacific. Think silty bottoms, rubble, and little pockets of brackish-to-marine water that change with tides and rain.

They are one of those tiny, easily-overlooked gobies that look harmless. They are not. Some Yongeichthys are known for potent skin toxins, and even if every individual is not equally nasty, you want to treat this fish like it is toxic every single time.

Setting up their tank

Give them a small footprint tank with lots of bottom detail. A 10-20 gallon works for one, but bigger is easier to keep stable and gives you more options for tankmates. They spend their lives down low, parking on sand, rubble, and under little ledges.

For brackish, I have had the best luck keeping them on the saltier end rather than barely-brackish. Stable salinity matters more than chasing a magic number. If you are already running a low-end marine setup, they adapt better than they do to yo-yo conditions.

• Substrate: fine sand or very small gravel so they can sit and scoot without scraping themselves up
• Hardscape: piles of small rock/rubble, shells, and a couple tight caves - they love wedge-shaped hideouts
• Filtration: oversize sponge or HOB with prefilter; gentle flow along the bottom, stronger flow up top is fine
• Cover: tight lid - gobies can pop out when spooked
• Salinity: pick a brackish range and keep it steady; use a refractometer if you can

I would not keep them in a brand new tank. They do better once there is some biofilm and micro-life on the rocks and sand. A mature brackish tank also tends to have fewer random parameter swings, which these little fish do not appreciate.

What to feed them

Plan on feeding frozen and live foods, at least at first. Many arrive skinny and picky. Once settled, some will take pellets, but I would not buy one assuming it will eat dry food right away.

• Best staples: frozen mysis, finely chopped prawn, enriched brine shrimp (as a treat, not the only food)
• Great small foods: copepods, amphipods, live baby brine, blackworms (if you trust your source)
• Feeding style: target feed near their hideout with a pipette or turkey baster

Small meals beat one big dump of food. These gobies are built to pick at small prey, and heavy feeding in brackish tanks can foul the water fast. I aim for 1-2 feedings a day, tiny portions, and I siphon leftovers if I miss the mark.

How they behave and who they get along with

They are bottom-oriented, a bit secretive, and can be surprisingly territorial for their size. Most of the time they just perch and watch, then do a quick dart to grab food. If you keep more than one, expect squabbles unless the tank is roomy with lots of separate hideouts.

Tankmates are where people get burned. You are balancing three things: the goby is small, it can get bullied, and it may be toxic. I stick with calm brackish fish that will not harass it or try to swallow it.

• Decent choices: small mollies (brackish-adapted lines), bumblebee gobies in similar salinity (watch food competition), quiet smaller monos when young (in larger tanks)
• Avoid: aggressive scats/large monos, fin-nippers, anything big enough to eat it, and highly predatory crabs
• Also avoid: delicate show fish you would be devastated to lose if a toxin event happened

Breeding tips

Breeding in home tanks is possible in the general goby sense (cave spawning, male guarding eggs), but with this group it is not a common, repeatable project for most hobbyists. Sexing is not straightforward, and getting a compatible pair is half the battle.

If you want to try anyway, set the tank up like a little goby condo: multiple narrow caves (PVC elbows hidden under rock work work great), lots of micro-food availability, and very stable salinity and temperature. Condition them heavy on meaty foods. If you ever see a fish camping a cave and refusing to leave, you might have eggs inside.

Common problems to watch for

The most common issue is simple starvation. They can look fine in the store, then slowly fade because they are not competing at feeding time. The fix is target feeding and choosing tankmates that do not mob the food.

• Skinny belly and hollow head: not eating enough or food too large - switch to smaller meaty foods and feed near the hiding spot
• Scrapes/redness on the belly: rough substrate or sharp rubble - smooth the hardscape and use finer sand
• Rapid breathing/hanging at the surface: salinity swing, ammonia/nitrite, or low oxygen - test immediately, add aeration, and do a water change
• Ich/velvet: can happen, especially after shipping - quarantine new arrivals and avoid temperature/salinity bouncing

The other big one is the toxin angle. Most of the time, nothing dramatic happens. But stress, injury, or death can dump nastiness into the water. Keep carbon on hand, keep up with maintenance, and do not let a dead fish sit overnight.