30gal Opae tank?

[Fishkeeper]

Hello all,
I have a Biocube 29 standing empty. I don't know the exact size of it, but it's somewhere in the 25-30 gallon range. It's a cube, about 18", and I want to fill it with shrimp.
As of right now, I have a cycled 3.5gal shrimp tank. I've had it up since January 2018, and I haven't had any breed. I think it's too cold- I keep my room in mid-low 60s. They're lively and colorful, and I had a berried one in April so I know I have at least one female, but they aren't breeding. You can't heat a tank that size safely, I know, but I DO have this much bigger tank.

Supplies I have:
About 3 cycled coral rock chunks covered in algae
A couple pounds of cycled sand, again, algae
A decent-sized bundle of macroalgae
10? shrimp
Some non-cycled Hawaiian coral rock (found on a beach, probably contains dead things)
Granite chunks
Reef salt

My plan so far is this;
1: add dead rock and about an inch of substrate to tank, fill with appropriately brackish water
2: add appropriate heater to keep water at about 76-78F
3: cycle tank until dead rock stops giving off notable amounts of ammonia or nitrites
4: add all materials from old tank
5: enjoy

There are no easily noticeable issues in there, right?

Will 10 (or I might get more) shrimp breed in a tank that large? They'd find each other when they wanted to breed, right?

I have a couple of nerite snails, currently in freshwater. Once the tank is cycled, I'm planning to acclimate them over. Anyone ever tried nerites with opae ula? I'm assuming there won't be any issues. Nor should the snails eat all the algae or anything- they've been living in less than 5 gallons of water and eating algae in there just fine, so they shouldn't put much of a dent in 30gal worth of algae-growing space.

Also, I have a lot of empty space to fill up. Any suggestions on where I can get things to fill that space? Safe and appropriate-looking rock, mostly, or some sort of synthetic material. I don't want to mess with lava rock and its potential contamination. Any good sources for dry reef rock?

[Fishkeeper]

I'm actually disinclined to scrub the rocks at all. They don't stink, so it's not like there's a dead crab in there or something, and anything small caught in the gaps will rot away while the tank cycles. Besides, my current tank was set up with rock from this same beach (washed up into the sun and dried out, not collected live), and it got some really interesting algae on it. I think some of the algae spores can survive being dried out, and I don't want to wash them off.

Your tank looks great! What number of opae do you find you can support? I read that tanks can hold at least 100 opae per gallon, which would make this an exciting prospect.

How did you cut the back plastic out without damaging the tank?

I know I won't see 10 opae very much, I just want them to be able to start breeding.

Okay, so they like more like goldfish temperatures. I can probably find a heater that can be set for low 70s. I can tell yours have been breeding well, because you have 600 in there!

[Dch48]

I have 16 in a one gallon tank. One did get berries but lost them in a couple of days. I've only had them for 2 months now. There is no heater or filter. There haven't been any losses and I have seen quite a few molts.

I would start out with at least 30 in a tank the size of yours.

Why can't you heat a 3.5 gallon tank? I have 2 of them and have no problems with heaters. One has a preset that keeps it at 78 and the other one has an adjustable that at the lowest setting keeps the water at 74.

[Fishkeeper]

The smaller a volume of water, the riskier it is to put a heater in. There's less temperature inertia, and many heaters are too powerful. A lot of the heaters that are small enough are cheaply made and prone to breakage. It's not that it can't be done, per se, only that it's riskier than I'd prefer to bother with. Plus, this tank is plastic.

[Dch48]

My tanks are plastic too. The one heater is 15 watts and the adjustable one is 20. You just have to not use too powerful of a unit.

[Dch48]

I agree with that for an Opae tank. You don't need a heater.

[Fishkeeper]

Okay, so it's unlikely that being in the low 60s all the time is stopping them from breeding? I wonder what it is, then. Maybe the light cycle is wrong.

[Afischer]

How often do you feed? Mine did not start to breed until I skipped feeding for 3 months.

[Dch48]

Low 60's may be an issue. Their metabolism slows down and maybe the breeding instinct lessens as well. I would think 68-70 should be a low point for an Opae tank so maybe a heater would be in order. A bigger tank like that warms up a lot slower when the ambient temperature goes up.

[Fishkeeper]

I haven't fed them yet. There's plenty of algae for them to eat, and there's so few of them. I didn't think it was needed.

Does anyone know if opae need to be physically kept from accessing a heater? They won't stand on it and get burnt, will they? I'd hope they'd have the sense to keep away, and I don't know what I'd do to keep them off. Heater guards are made to keep fish off, they have gaps (which are necessary for water flow) that opae could slide through. But they can't be that dumb, that they'd stand on a heater and overheat without ever thinking to move away. Decent heaters don't get "burn you on contact" hot, you have to maintain contact for a bit to get an issue.

[Ula Hula]

Mustafa said he had breeding in tanks that he kept outside in San Diego where the low temperature was 39 degrees! These are some hardy shrimp indeed.

[Fishkeeper]

I'm going to get a heater, and just for insurance, I'm also getting one of those shutoff probe things that turns off whatever is plugged in if it gets past a certain temperature range. That way, if it fails, it'll fail in the direction that results in chilled shrimp and not cooked shrimp.

Even if they don't need a heater to breed, the heat will definitely speed up their breeding. Warmer water means warmer shrimp bodies, which means faster shrimp metabolisms, which means faster breeding and growing. Which, given that I hope to eventually have a couple thousand shrimp in this tank (and I know that'll take a long time), is ideal.

[Mustafa]

Mustafa said he had breeding in tanks that he kept outside in San Diego where the low temperature was 39 degrees! These are some hardy shrimp indeed.

Just to give it a bit of context, the shrimp were breeding before it was that cold and the larvae were still floating when it got colder. Also, keep in mind that temps in socal don't stay that low. The days will still be in the 60s at least in the winter. The 39 mentioned above is basically the low of the day for only about an hour or two at most and then the temperature rapidly rises. And 30s are still pretty rare during most winters where I live. 40s are about as low as the low for the day gets on most winter days. The above mentioned tank was a 10 gallon tank so it adjusted to the outside temperature rather quickly. It's definitely not ideal though. Nowhere near....

[Mustafa]

I had an outside tank that had it's heater accidently unplugged for a few days. All the opae were on their side on the bottom. Not moving. I could tell btthey were still alive so I plugged the heater back in and within 5-6 hours they were swimming once again. No deaths. I doubt that the opae would breed in Southern Ca in the winter, still too cold.

That's because the temperature dropped rather suddenly. If the shrimp are out there and have time to adjust to the lower temps gradually with the changing season, then they won't drop on their sides and almost die. They will actually even continue picking at the biofilm until about the mid to higher 40s, just a lot more slowly...like slow-motion picking. At lower temps they just sit there and you can tell they are just waiting and "hoping" for better times.

Another extreme temp sidenote...on the high end they can take temps to about 102F for a short time. At 104F they start dying. You'll see that they are not feeling too happy in the upper 90s because instead of picking really quickly at the substrate, they start swimming around the tank. (And no, I did not raise the temps that much on purpose.)