Frankly - this is the crabby old conservative speaking from his soapbox - so, FWIW:
I just do not think that a 29 is big enough for 2 F-8s and a pair of knights. I might do a pair of BBGs as they are smaller, but either setup for 4 fish I would use a 40-long by preference. I will not go below 15 gallons of water per F-8 again, with zero other fish overlapping that volume. But I am neurotic about water volume and stability.
I would also bet that if I started a new F-8 setup, I could get the average lifespan up to or above 20 years. It is a shame that it took me so long to learn how to keep fish properly. It really is easier than the ways I used to do things. We get so soon old, and so late smart.
Is it a overcrowding issue or a water quality issue? If it's overcrowding then I won't do it, but if it's purely water quality I think I can maintain relatively good WQ with weekly changes and perhaps using an old 10 gallon to setup an algae scrubber/sump.
I have two young F8s and 4 baby balloon mollies (all the mollies are females) in my 29 gallon. The mollies are very entertaining and are great at cleaning up the bits and scraps left behind after the puffers eat crushed snails.
Per RTR's advice a 29 isn't big enough for 2 F8s and 2 Knights, so for a 20 gallon... I'll leave RTR to answer that. I'm sure he'll be back.
So RTR, would 2 F8's and a 4 BBGs be overstocked as well? How do bottom dwellers (knights) and mid-upper level fish (F8s don't lurk on the bottom, do they?) overcrowd each other? It's not that I'm not taking your advice, just looking for more information. Right now the only definite thing I'm going to do is return the dragon goby and pick up the two F8s the LFS ordered for me. They also added a few BBGs to their order as well, since I said that was what I intended on putting in my tank. They do have a few knight gobies though, the clerk mentioned those to me as an option (but he really pushed the dragon goby, they must sell poorly?).
IME It is largely a water quality issue, but it is also a water volume issue. It is not possible to compensate for a too-small tank by extra water partials. As I said, I do consider 15 gallons the absolute minimum water volume for one F-8 these days - and that is a measured 15 gallons of water, not a 15 gallon tank (which will not hold 15 gallons) for the puffer alone. In a 20, I have kept a single pair of BBGs with a single F-8, but now I would use a larger tank. BBGs do not bother F-8s IME, but may someday get eaten. I do tend to overfeed a bit, and the BBGs are good at leftover clean-up, But my F-8s get snails as a stable diet and other foods only as treats, so clean-up is a small chore. Also I use OE-RFUG for low-end BW to keep the alkalinity as stable as possible for F-8s (as important as the salt content IME), so any scraps stay on the surface of the substrate and are easy to get out of the water.
It seems bizarre that fish which in nature can swim from one density to another at will, and inhabit environments with constantly changing water conditions, should react so well to higher than normal or natural stability in captive specimens. But my goals in captive specimens are either life span or breeding. In the wild breeding is everything. In captivity it markedly shortens lifespans. We cannot yet breed F-8s or GSPs readily. When we can breed them, we will have to trade off reproduction with shortened lives for the breeders. Or a perhaps acceptable lifespan reduction with one or two breeding per year for any given fish. But we do not yet face that issue. I hope that we do eventually.
No volume of water partials will support a tank which is overstocked unless the tank is automated flow-through at or above about 15% per day changes at low stocking levels. Over-stocked tank may require multiple total volume changes per day. The math gets tricky on flow-through systems, the volume to waste is larger than the percentage actually changed due to mixing. And the setup must still provide sufficient current and space for exercise - under-exercised puffers get fat and that in and of itself shortens their life. They need the right food but not too much food. They need water current to promote exercise, but also areas of lower current and some apparent shelter for naps and sleep. They need a visually complex environment if they are hunter predators, as both F-8 and GSP are, to give them territories to be explored repeatedly for food - a natural behavior which is not lost when they are well fed by their human servants.
Very, very few "pet" fish have their needs met. Most are stressed (mixed and community* tanks); over or inappropriately feed (leading to malnutrition and/o obesity); few have sufficient swimming space and current (3-4x their max adult length front to back space and 6-10x their max. length for end-to end swimming space, with appropriate current to match. All shouild have minimum 50% weekly water volume changes, with mechanical filters rinsed at the same of more frequent intervals - IF those weekly partials maintain the total water profile within one 50% partial of the tap or other source water parameters: for calcium and magnesium hardness (GH), carbonate/bicarbonate hardness (KH), pH, phosphate titer if unplanted, and nitrate levels also if unplanted. Ammonia and nitrite levels should always be undetectable by hobby-level test kits. If all of those are not close to the original water parameters, then the tank needs to be larger or changed more frequently or the stocking and/o feeding analyzed to correct the problem (s).
*Community tanks, a.k.a. Noah's Ark tanks and confetti tanks, are generally maximum stress situations for fish. Few fish in the wild travel in mixed schools with other families of fish. Even if they share a certain river or lake, they tend to each have their own ecological niche - which they may share with own kind in close assembly as schools. or on their own as solitary specimens other than for breeding.
F-8 both nap ans sleep on the substrate, so yes, they do impinge on bottom dwellers and vice versa. Knights are a bit feistier than BBGs IME, so need more room. BBGs don't need as much space as Knights and are more likely to be eaten themselves. GSPs may eat either species.
More than you wanted to know about my philosophy of tank-keeping. My object is to give whatever wet pet I am keeping the best set-up for their needs, and to keep it as stable as possible for as long as possible. Some years back, I decided to start doing things right by my own standards, and if others did not like it, that was not my problem. I have been delighted to find out that my fish do tend to agree with me. They just keep on trucking, year after year and decade after decade. My tank room could be called an assisted living facility as it has so many elderly but healthy fish . The kicker is that the tanks are also easier to maintain. Color me happy.
I know you are probably getting very tired of answering my questions, but I only have one left. Is a 29 gallon (30x12x18) adequate for two Figure 8 Puffers? No tankmates other than snails and shrimp to be restocked a few times a week.
I would say no. As i asked a similar question recently and got that as an answer. Thats approx 90 litre right? As in 90l total water volume when set up approx.
It's amazing how easy maintenance is. If done regularly and thoroughly
With the filter canister, minus substrate volume, minus one hollow resin decoration, I'd say I'm getting about 26 gallons of water volume. So about 100 liters of actual water in the tank. The standard dimensions I found for a 90 liter tank states it's 60 centimeters long (23.6220472 inches) where as a 29 gallon is 30.5 inches long. Much more swimming room. The depth (front to back) of a 90 liter is 35 centimeters (13.7795276 inches) and so is a bit larger than the 29 gallon's 12.75 inches there. As for height, the 90 liter is 45 centimeters tall (17.7165354) and the 29 gallon is 18 inches tall.
So, with that said, a 90 liter is significantly shorter than a 29 gallon aquarium. I know Pufferpunk kept a pair of F8's in a 29 for a long time, along with various other tankmates. I'm almost certain I'll go with a puffer-only tank now, and that I'll put in a pair. I'm just looking for RTR's advice; he has already changed my plans, and my perspective on the hobby a bit. He may yet convince me to house one lone Figure 8 in my 29, or even to go FW and put in a Dwarf, maybe two if I do extra waterchanges.
I would be astounded at any nominal 29 gallon which could hold that much water. I only have one 29 in operation, and it is heavily planted - so has much more substrate them F-8 tanks get. My tank contains 21 US gallons of water when full to a normal working volume. there are no rocks or other hardscape elements.
Nominal tank volumes are pipe dreams. My 55s hold 41 and 42 gallons respectively, again both are planted so have deeper substrates than BW/SW tanks. Again there are no hardscape elements. I only measure water once - when I set a tank. I do like to know, in fact I am neurotic to know, exactly what I am working with.
That said, in answer to your question: Will two F-8s live in a 29 with no other fish. Yes, they can and will. Will they live to their expected lifespan average of 15 years? Probably not. That housing would be minimum housing, and would need plenty of hardscape (a high multiple of just one artificial mangrove piece) and the water volume will be reduced by that. Plus the dominant fish will live longer than the sub-dominant fish. Hunter predator puffers do not spend much time in the company of their own kind in the wild. Con-specifics are stressful. If you kept the same two fish in adjacent 20-longs, they would last longer. At least they do for me. Hunter predators are by nature aggressive at intrusions into their feeding territory.
Some fish fry, housed together in the LFS will form apparent bonds with conspecifics which may be quite lasting in captivity. As we have no external sex differences which are reliable before sexual maturity and not always then, we have no way of knowing their sexes. I suspect but cannot show that such lasting pairings are likely females. I also suspect the male-male pairings are quite rare long term in captivity and male-female pairs are equally so. We do not make any efforts to provide these fish with their normal hunting territory as seen in the wild (which will be highly variable in any case, and are likely seasonal). Our housing of these fish (as well as most others) is arbitrary, and IME insufficient. Relatively small increases in water volume per fish in captivity can have huge effect on the particular fish involved. As does singleton housing for non-schooling fish. Anyone can - if they wish - test those set-ups. To date I have never had anyone refute any of my observations on these fish. Most co-house fish IME will live about 1/2 to 2/3 of their singleton lifespan. And do remember that 'Singleton' means just that - one fish in any ecological niche per tank. They are not affected by tank-to-tank visibility of conspecifics - which rather surprises me. Display, without physical contact and/or pheromones in the water apparently does not affect lifespan or general health and activity, and may even be beneficial. But the tank in question is still minimum and sub-optimum housing for two fish. Over these fishes lifespan, doing more partials per week and such bandaid techniques are far more expensive and troublesome than providing more space up front. Think about the cost difference between a 29 and a 40-long, and spread that out over 15 years. The inital cost difference is trivial. The operating cost will be a bit higher, but less than the cost of doing two large partials weekly, and far less bothersome.
My professional life was spent developing production techniques and optimizing them. I cannot ignore such things in my hobby world either. I have to play with optomising my tanks as well. If I can make my tanks operate better and longer, why would I not do so?
