Is it possible that you are overfeeding? I did that with my first aquarium, and had lots of problems. I now feed 3-4 times a week and everyone seems happy.
As far as the mineral deposits on the glass; that's hard water (we have that issue here in San Diego, too). I have started to use half RO water in my water changes. The problem is that those minerals actually etch the glass if you don't get them off. Try using a scraper and then a plastic scrubby and white vinegar to remove it. Good luck!
I'm coming in late here but I did read through the thread. Honestly with as overwhelmed as you are you need to stop working on the new tank and solely concentrate on the other two tanks that have fish. The new tank can sit and run with a filter for now. I know you have the bug of multiple tank syndrome but you need to get a good understanding of how the tank cycles and works by perfecting it on the smaller tanks. Then move onto the new larger tank.
The reason your losing fish is because of the tanks not being cycled. And do correct me if I'm wrong about those two tanks not being cycled. Many fish just aren't hardy enough to withstand the cycling process. I also think I read on one of your threads that you have an ich problem to boot?
Something else, you can save so much money for supplies and gas if you start shopping online. Equipment is normally a lot cheaper and if you shop around you can often find free shipping.
Also when it comes to feeding... you don't need to satisfy every fish at every meal. Feed once a day and vary your daily feedings. Say flakes one day, tubifex worms one day, and so on. Algae wafers can be used as a supplement not as an only daily meal. Fish will eat when they are hungry. Fish don't even need to eat daily so don't worry they will be fine.
I'm coming in late here but I did read through the thread. Honestly with as overwhelmed as you are you need to stop working on the new tank and solely concentrate on the other two tanks that have fish. The new tank can sit and run with a filter for now. I know you have the bug of multiple tank syndrome but you need to get a good understanding of how the tank cycles and works by perfecting it on the smaller tanks. Then move onto the new larger tank.
The reason your losing fish is because of the tanks not being cycled. And do correct me if I'm wrong about those two tanks not being cycled. Many fish just aren't hardy enough to withstand the cycling process. I also think I read on one of your threads that you have an ich problem to boot?
Something else, you can save so much money for supplies and gas if you start shopping online. Equipment is normally a lot cheaper and if you shop around you can often find free shipping.
Also when it comes to feeding... you don't need to satisfy every fish at every meal. Feed once a day and vary your daily feedings. Say flakes one day, tubifex worms one day, and so on. Algae wafers can be used as a supplement not as an only daily meal. Fish will eat when they are hungry. Fish don't even need to eat daily so don't worry they will be fine.
Feed only once a day period. Pick a food and feed that and add a shrimp pellet for the corys. If the algae wafers are only for the pleco then feed one every other day "at night" after the lights are off. Pleco's mainly eat at night. You don't need to feed him daily either.
Feeding is the least of your issues right now. You need to concentrate on getting the two smaller tanks cycled. I've been in this hobby since the 70's and you've got too much on your plate with the two small tanks. I'm sure you don't want to hear to leave the 75 alone since you have it now but that is the best thing you could do for yourself and the other tanks with fish.
Fish are the worlds best beggers. Just because they beg for food doesn't mean they need it. Goldfish actually don't have stomachs so they actually do better with a couple SMALL feedings a day. They also do better with at least one meal a week of pea's only.
You cook a few frozen peas, let them sit and cool, then pop the outer shell off them, and feed to the GF. This helps keep them from being constipated which can be a problem with fancy goldies.
Algae tablets wouldn't be a supplement for Pleco's but part of their main diet. Some people use them for other fish in which they would then be a supplement. You need to get a couple cheap little veggie clips so you can attach zucchini strips to the tank glass at night. I cut 1/4" or slightly thicker strips of zucchini, microwave them for 10 seconds, then put them immediately under cold running water to stop the cooking process. Put them on the veggie clips and hang in the tank. It may take a few tries to get the pleco used to coming and eating them but it is very good for them. I feed this every other day.
You can weigh the veggie down by rubber-banding it to a stone until you get your clips. I got mine online because I thought $8 for a plastic clip at the lfs was insane!
speed, Thanks you for starting this thread, I started y 1st tank 2+ years ago I I still learned things from this thread.
Also remember that no matter how dumb you feel you beat most of the population because you are smart enough to ask for help.
My 1st tank was planted and I went thru several fish but I still kept most of them live until just recently, I just got too busy with College and neglected my tanks, Shame on me.
I also used a bacteria starter to help cycle them that made a big difference. At least I feel like it did.
Also I know that someone recommended Prime as your dechlorinator. I will go further and recommend a set of "Good" quality foods too. I used the cheap brands of food and had the cloudy water issues and sick fish all the time, switched to Omega one veggie tabs and color flakes and Spectrum mini pellets with garlic and I saw a major improvement in my fishes. the cheapie veggie tabs would sit in the tank and mold in 24-36 hours, but the Omega One's never mold, never last that long anyway.
as for water changes I would recommend a faucet powered water changer too, especially on the 75. you just connect it to the sink turn on the cold cold and it will siphon water out of the tank down to what ever level you need, the adjust the temp to match you tank, and Prime, and flip the lever on the changer; and it will quickly fill it. it you want to save on water, only use the changer to fill, then at Harbor Freight or Northern Tool they sell small 120 volt utility pumps, connect the "out" end to garden hose out the window or door and on the intake side you can buy a female garden hose end, that you can attach to your gravel vacuum hose and just use that to drain the tank, and/or water the garden, plus it is pretty good fertilizer for the garden/yard too.
Your right you are going through the cycle. The best option would be re homing your fish and doing a fishless cycle. People are saying change water to reduce stress on the fish. During the cycle you would normally avoid changing water until only nitrates are present as they really only reduce through water change. You are right though that changing water often will make the cycle take a long time