New setup/need advice

I just went from a 40 to a 75 gallon after 25 years and I need to add more stuff. (its very empty looking in there now) I looked at the rocks and such at the fish stores but was wondering if you had any ideas about building taller structures for the fish I have.(materials etc.) and other choices as to where to purchase.
I have 25 year old clown loaches (6) 1 headstander (same age), firemouth ciclids (pair) also been thinking about adding some fish that would school in the middle if possible. any ideas? Thanks alot!
Cheryl

I will try to get some pictures when tank looks cool. I never expected them to live that long.(they started in a 10gal, than have been in the 40gal for 22 years) hopefully I won't kill them off by moving them to this new 75 gal. setup (so far so good)
I have been reading about clown loaches and they can live 50+ years if treated right. I may have to will them to my daughter
Cheryl

Their sizes range from about 4 inches(smallest guy) to 6 inches long. aprox. 2 inches high. Not real huge, I think the 40 gallon never allowed them to grow any further than that. Now, that may change.... I got them all at once and didn't lose any in all those years so their a tightknit group of fish. I also bought a headstander and koolie loach that same day and their still around too. I was very dilligent about them water changes....I had a professional guy come in and setup new tank, so old undergravel filter is in new setup for awhile till everything is ao.k. So far they look happy.(ph nitrite & ammonia is at proper levels) I am not real familar with trickle/wet/dry filters so this part is all new stuff for me.Any advice there would be helpful and advice on neat things to add etc..... Love the fact that the heater isn't an eyesore in the background! And now that theres alot less gravel, how do you keep the plants from floating around????
Cheryl