How Badly Overstocked Is This Tank?

[Doomchibi]

My mother has a 90 gallon tank set up in our living room and over the years she has had some questionable things and bad mixes in there, yet right now I think it is at it's worst. She ignored me when I told her it was a bad idea, but a few months back she added about 10 male and 5 female fancy guppies. Now, the current stock of the tank...
 
4 Black Skirt Tetras
1 Neon Tetra
1 Headlight Tetra
1.. What appears to be a blackline raspbora but the black line has turned into a big black patch going down both of it's sides
4 Black Mollies
2-3 (not sure) balloon mollies
approximately 20-25 hybrids of what look like balloon mollies / mollies ?
1 bristlenose pleco
1 (common?) pleco though she has had it for years and it has stopped growing around 8", so it may be something else.
 
and.. *drumroll* Approximately 40 adult, 50 juvenile and 30 baby fancy guppies, along with 10 of the adult females being very close to giving birth. (every few days we find more babies in the tank)  Grand total..  120 guppies, though there could be as many as 150. (I cannot count them all because they move too quickly, and I know at any given time there could be 30-40 or more guppies in the plants, décor, etc and I will not see them.)
 
 
I have tested her tank before and she had no ammonia, no nitrites, and a LOT of nitrates, I believe around 50? When I told her how badly the water needed to be changed she started doing very small changes every week or so, so at least now the gravel is a bit cleaner but it is still pretty bad.
 
She also grossly overfeeds the entire tank, I would say she goes through about 1/8 of a cup of food every day on her guppies alone. Since they all hang around the top, her other fish only get the odd bit that manages to get through the angry mob as I call them. Because the guppies "beg" a lot, she assumes they are always hungry and so I have seen her use as much as a 1/4 cup of food in the whole tank during a day. She adds it until it looks like a snow globe. Sometimes I wake up early a few days in a row after a massive feeding so I can be the first one to turn the light on and feed them very lightly, so when she wakes up I can tell her it is already done and if I am lucky she will leave them alone.
 
I would like to be able to help her but she will not change anything unless it involves adding more fish or unless I am extremely convincing and what I tell her makes sense.
 
I know her tetras need to be in larger groups, they were at first but over time they have died off. Her neon looks miserable but I am very hesitant to mention it to her because she will go out and get another 10 of them.
 
 
How many guppies are suitable to keep in a 90 gallon tank along with the other stock? Have any of you ever had any luck convincing a very stubborn person that their tank needed to be changed? If so, I would really like some advice. I see this tank every day and every time you walk by it, everything moves and suddenly there is a sheet of guppies at the glass wanting food. It's so sad but not my tank so I can't outright change something

I am thinking maybe if I can show her they are unhappy she would let me take the females so they would stop having babies, and she may do that simply because she thinks the females are too plain.
 
Any tips or experiences would be helpful. Thank you all

 

[Raticataticus]

Okay call me crazy (evil) but is there any way that maybe you could remove a few of the guppies here and there and either sell them or give them to a local pet shop or something? If you just remove a few of the plainer ones here and there she might not notice. 
 
If she's really as stubborn as you say that she is I doubt you'll be able to convince her to do anything.

 

[guppy2002]

try to have HALF as many male guppies as females. thats all i have to say.

 

[Fishmanic]

with all that feeding,  she may eventually get ammonia and nitrite spikes

 

[Blobfish]

This is maybe a bit off the wall but would showing her pictures of really amazing aquascapes help?  I'm thinking if she saw some really stunning ones she might get interested in making over her tank and perhaps in the process you could weed out some (or a lot) of the fish.

 

[Doomchibi]

At the moment she doesn't have a single living plant in her tank and i'm finding it hard to believe she would be interested in trying to keep them unfortunately. She thinks they are a lot of work / too difficult to keep alive. She loves how they look, she just really doesn't want to put more effort into keeping the tank. Are there any low maintenance plants you can suggest? How many guppies would you say are the most she should have in the tank with the other fish? Maybe I could point out that if she had fewer guppies she wouldn't have such a filthy tank all the time.. She spends hours multiple times a week vacuuming the gravel but it's still not enough when you consider all the uneaten food she's adding every day too. 

 

[the_lock_man]

try to have HALF as many male guppies as females. thats all i have to say.

 
With that many guppies, you don't need to worry about the 1:2 or 1:3 ratio

 

[ech0o]

Not saying it isn't overstocked, but if it was fully planted it wouldn't really be that bad.   I have seen 120's fully planted with near 300 guppies in them that I wouldn't consider drastically overstocked, guppys dont really go by the general rule of 1 inch per gallon IMO.    Luckily the mix of fish you have in the tank is good, there are no serious predators in there, molly and guppy go together quite well.    I would say start selling guppies, put an add on craigslist, and try to keep the population thinned, as they will just keep reproducing over and over and over and over.     And get some plants in there for sure!!!!

Also I would say up the schools of tetra that you have less then 5 of

 

[Blobfish]

Vallisneria would be good for the nitrates and is very hardy.  You could buy her a fish as a present, something like a firemouth or a nice predatory catfish