[Auballagh]

Ahhh yes.....
A bigger tank. And, if I understand your situation correctly, the problem is your second floor siting of the aquarium, correct?
Weight.
So, let's peel this onion down a bit, and look at this thing a little closer.
For one, I believe you are almost certainly correct in wanting to limit the size of your FOUR FOOT LONG Aquarium. Moving up to a 90 gallon, or certainly The 'Big One' at 150 gallons, would be putting a lot of strain on a four foot long section of 'footprint' space on the floor. And esp. those load bearing floor joists and such located under the floor beneath the tank. Agreed. That is, too much water/weight distributed over that FOUR FOOT LONG section of floor!
So then, let's look closer at the SIX FOOT LONG aquarium. That extra two feet of length will distribute and place the weight over a larger area. In this case, almost certainly spreading that weight over two or possibly even three additional floor joists. That is, those load bearing structures sited beneath the aquarium.
So, it's possible (I'm NOT a structural engineer), that the SIX FOOT LONG, 125 gallon aquarium will have approximately the same or possibly even SLIGHTLY LESS weight pressing down on each of the floor joists below it - than the four foot long tank will. More floor joists = better distribution of the weight.
So, by using that metric.... have you seen one of the most amazing aquariums ever on Cichlid Forum? That would be - of course - the EIGHT FOOT LONG, 135 gallon aquarium owned and operated by none other than the (world famous) @Sinister-Kisses! That really long aquarium would DEFINITELY place less weight on the floor joists located beneath it, than this 75 gallon tank you are looking to get! Kind of crazy I guess, when you think about it....
-
Recommendation: Get a 6 foot long, 125 gallon tank. And, if you're really nervous about the overall weight? Reduce that weight a LOT by going with an acrylic tank (A single person can pick up and move around an empty, 125 gallon ACRYLIC aquarium). Or, if you REALLY wanna be safe? Go even longer, with an EIGHT FOOT LONG, 135 gallon acrylic tank (probably custom made = $$$).
And well, if you actually go with something that size? Well, we will definitely be having a VERY different stocking discussion than we are now having about a 75 gallon aquarium!

 

[TotoroTony]

My apartment is a Co-op in which I have to abide by rules of the building management. I utilized a 75 gallon tank before and had no structural stress issue. Even though a 6 foot 125 gallon tank is ideal the extra 400 lbs could be dangerous for a 78 year old building. But my apartment is not big enough as other furniture inhibits bigger size. If I eventually move out and acquire a ranch style house instead of current apartment, then I go big with 180 gallon tank with my favorite fish without issue other than maintaining clean and healthy environment for my fish buddies. Thanks for insight about ACRYLIC TANK as a weight reduction.

 

[Auballagh]

Normally, I'm really not much of a fan of acrylic tanks. That plastic scratches super easily. Plus, I'm cheap. A new acrylic 75 gallon tank, is gonna set you back 2 to 5 times the cost of a new glass aquarium.
So, if you're limited to 75 gallons anyway, and the glass one you had before worked fine in your apartment and complied with the rules of the Co-op? I would personally recommend just purchasing another glass, 75 gallon aquarium.

 

[TotoroTony]

That may occur in the future after the fish start sprouting. I do have space in another room which is an option.

 

[Sinister-Kisses]

have you seen one of the most amazing aquariums ever on Cichlid Forum? That would be - of course - the EIGHT FOOT LONG, 135 gallon aquarium owned and operated by none other than the (world famous) @Sinister-Kisses!

Well thank you for that high praise!

 

[TotoroTony]

No issue for the support of my tank as it was professionally done by my local fish store. He stated my apartment building has solid concrete walls and floors which easily could of handled at least 125 gallon tank or more. Furthermore he also confirmed that I picked the best location in the room. My tank is crystal clear water on it's third day of cycling. Added Fritz Turbo Start 700 with natural bacteria to speed up the cycle. Will test water tomorrow as there were small traits of ammonia & Nitrate today. Nitrite was zero. Want to eliminate the remaining ammonia before adding a couple of fish.

 

[TotoroTony]

My tank is now safe for fish. Ammonia & Nitrite are "ZERO" and Nitrate is low at 2.5 ppm. Will start adding a few fish perhaps today as the Fritz turbo start 700 has expedited the cycle. I will continue to monitor levels and keep feeding minimal. Fish at baby stage are not large bio-load issues.

 

[TotoroTony]

My first "four" Cichlids have been introduced to the tank. Firemouth, Rainbow, T-Bar & Blue Acara. All babies around an inch. Apparently no local fish store has a "Regular Jack Dempsey" available. Therefore I ordered one online and should arrive next Monday or Tuesday. Healthy Silver Dollars are also in short supply. Perhaps aquarium fishes are not as popular in New Jersey as a few stores I previously utilized are closed permanently. Shame as small businesses suffered the most during this pandemic. I think I shall just keep "FIVE" fish for this tank. If the CICHLIDS want more action in tank will consider the Silver Dollars, Giant Danio or a large BARB type as dither fish. ****, should of bought a 125 gallon tank...lol

 

[TotoroTony]

The only issue buying online is that shipping & handling costs much more than actual one fish by far.

 

[TotoroTony]

Mr. Jack Dempsey received today quickly from California by [vendor name removed] in good health. My fish are now completed with Five rascals.

 

[TotoroTony]

One week after fish introduced to tank and great water test results. Since inception of tank lowered PH from 7.6 to 7.3 as New Jersey water is high alkaline and wanted my water closer to neutral. Ammonia & Nitrite remain at ZERO. Nitrate is low at 3.0. Water is crystal clear and fish are happy and active. I have been diligent to not overfeeding with one feeding around 7 am and another at 3 pm and they gobble up their flakes in less than a minute. Five CICHLIDS is perfect for this tank.

 

[TotoroTony]

First casualty is "Firemouth" passed away. Apparently a weak fish from fish store as no other fish in tank affected. Replaced with a "Polar Blue Parrot Convict" which although a HYBRID bred by man since early 2000's is a hearty fish and beautiful. Adapted well with my tankmates: Jack Dempsey, Blue Acara, Rainbow & T-Bar Cichlids.

 

[TotoroTony]

My 75 gallon tank is now completely cycled with 6 fish. Two RAINBOWS (one added), newer bigger FIREMOUTH, Jack Dempsey (female), Blue Acara and new electric blue acara. The Firemouth is a male and appears to be "KING" over female Jack Dempsey. Love the way the "ACARAS" swim as they glide so smoothly. Firemouth, Jack Dempsey and 1 Rainbow are in the 3 inch range now. Rainbows are the passive fish of community. Convicts were evicted as they were too disruptive.

 

[Auballagh]

Sounds good.
Oh, and.....

Replaced with a "Polar Blue Parrot Convict" which although a HYBRID bred by man since early 2000's is a hearty fish and beautiful. Adapted well with my tankmates: Jack Dempsey, Blue Acara, Rainbow & T-Bar Cichlids.

Jelly Bean Convict stuff. And, as we have all recently learned from info shared out by @Mr Chromedome , they are NOT hybrids. So, I'm not too surprised you experienced some fiesty-ness with those Cichlids. As you discovered, your Blue Polar Convict decided to NOT read any of it's own 'press'. And well, despite how sort of goofy they look (and swim, etc...), those little beasts still feel (and act) like they are a FULL UP Convict Cichlid, Raawwwrrr!
A Central American Bad-*ss, yo'.

 

[TotoroTony]

Added another Blue Acara to keep other one happy. So in general have 7 fishes.
Two rainbows (3 & 2 inch). Two Blue Acara (2-2.5 inches). One Electric Blue Acara (1 inch). One Firemouth (3 inch Male) & One Jack Dempsey (3 inch female).
Gotta make nice hiding spot for little electric Blue acara since he is an inch smaller than rest. Got a mini-log in which only he/she can fit it but he is smart and hides near driftwood and plants most of time. Hearty appetite for little bugger too.

 

[TotoroTony]

Firemouth booted from tank. Too disruptive to others. Back to six only.

 

[TotoroTony]

Hopefully final cast in my fish tank. Jack Dempsey traded for 3 Electric Blue Acara. My tank will have TWO Rainbow Cichlids and FOUR Electric Blue Acaras by the end of the week. Their personalities are better suited with J.D. They all should be within a inch of one another at maturity as the Electrics are a bit smaller than the pure breeds.